CONTENTS Introduction Disk Driver Basics Driver Gestalt Secrets of the Partition Map Non-512 Byte Block Devices Large Volume Support How the ROM Loads SCSI and ATA Drivers Loading FireWire Drivers Chaining Drivers and Patch Partitions Disk Drivers and the System Heap PowerPC Native Disk Drivers Installing and Removing Drivers and Drives Close and Purge File Exchange (né PC Exchange) Private Control and Status Requests Read-Verify Mode Color Icons Target Mode Disk Driver Power Management Summary Downloadables

This Technote is both a summary and review of existing disk driver information and a description of disk driver features that until now have not been generally documented. This Note is directed at developers of disk drivers and disk formatting utilities. There is also a section specifically aimed at application developers who need to operate on disks directly.  Updated: [Nov 22 1999]

Introduction

The Mac OS disk driver architecture has not been comprehensively documented since Inside Macintosh II (1985). In the intervening years, disk technology has changed radically, from 400 KB floppy disks to FireWire, visiting two different SCSI Managers and four versions of ATA Manager on the way. Many of these technological changes have been accompanied by architectural changes for which the documentation is in obscure places, was not generally released, or was just never written.

The technote is an attempt to rectify that oversight. It serves both to bring together the existing documentation and to fill in the missing pieces. You can use this technote as either a reference, an introduction to writing disk drivers, or just to bring yourself up-to-date on the latest disk driver advances.

If you are new to Mac OS disk drivers, you should start with the Disk Driver Basics section. If you're already familiar with the basics of the Mac OS disk driver architecture, you may want to start with the two high-level summaries, one for disk driver writers and one for application developers.

Existing Information

The existing documentation for disk drivers is scattered through many different Apple documents, interface files, and code samples. The section classifies these references based on their usefulness.

Core References

These large works cover information that you will definitely need in your driver. Don't start a disk driver without being familiar with these works:

• Inside Macintosh: Devices, SCSI Manager is the core reference for the classic SCSI Manager programming interface, introduced with the Mac Plus. It also describes the Apple partition map format, used by all Macintosh computers since the Mac Plus.
• Inside Macintosh: Devices, SCSI Manager 4.3 is the core reference for the SCSI Manager 4.3 programming interface, introduced with the Quadra 840av. All SCSI drivers written today should use the SCSI Manager 4.3 programming interface.
• ATA Device Software for Macintosh Computers (previously known as the ATA Device Software Guide) is the core reference for the ATA Manager, which allows you to find and control ATA devices connected to the computer. The "ATA Driver Reference" chapter offers a useful summary of the Control and Status requests relevant to a modern Mac OS hard disk driver, although some of the information is inaccurate and has been updated in this document.
• ATA 0/1 Software Developers Guide is a supplement to the above, and describes the changes required to support device 0/1 (master/slave) on ATA buses.
• Inside Macintosh: Files describes the drive queue, a key data structure used by all disk drivers.
• Technote 1041, "Inside Macintosh: Files Errata" comprises corrections to the core Inside Macintosh: Files document.
• The Shared Device Access Protocol specification.
• DTS sample code RAM Disk implements the basic framework for a disk driver. Unfortunately, it does not demonstrate how to handle requests asynchronously, which is one of the trickiest things to get right in a disk driver.
• DTS sample code TradDriverLoaderLib shows how to correctly install a Mac OS driver 'DRVR'.
• DTS sample code SCSI Driver Example demonstrates a fully fledged SCSI driver that supports both classic SCSI Manager and SCSI Manager 4.3. It is a useful sample, although it has decayed a bit in the years since it was last updated (1994).
• DTS sample code ATA_Demo demonstrates how to read blocks from both ATA and ATAPI disks.
• "DriverGestalt.h" (from the latest Universal Interfaces) always contains the most up-to-date list of Driver Gestalt selectors.
• The MoreDisks module from the DTS sample code library MoreIsBetter contains a comprehensive list of all the currently defined disk driver Control and Status requests, and where to get more information on how to support them.

Additional Information

These smaller documents contain information that supplements the above in certain key areas.

• Technote 1098, "ATA Device Software Guide: Additions and Corrections" is the latest errata for the ATA Device Software for Macintosh Computers.
• Technote DV 17 Sony Driver: What your Sony Drives For You documents the Control and Status requests supported by Apple's standard floppy disk driver. This is a key reference for disk driver developers. Floppy disk driver writers should also read the "MFM Disk Device Driver" chapter of Apple Logic Board Design LPX-40 Developer Note (hardware developer note), which includes information on floppy disk Control and Status requests that is missing from DV 17.
• Technote DV 22, "CD-ROM Driver Calls" documents the Control and Status requests supported by Apple's standard CD-ROM driver. This is a key reference for CD-ROM driver developers.
• Technote 1104, "Interrupt-Safe Routines" answers the perennial question, can I do X at interrupt time?
• Technote 1067, "Traditional Device Drivers: Sync or Swim" addresses a common misconception of device driver writers.
• Technote 1040, "Write Cache Flushing: Techniques for Properly Handling System Shutdown" describes how disk drivers should handle system shutdown.
• Technote ME 09, "Coping with VM and Memory Mappings" is probably the best place for information on ensuring that your device driver is compatible with virtual memory.
• Technote 1094, "Virtual Memory Application Compatibility" contains a description of the Mac OS VM architecture as a whole, which is useful background material for device driver writers.
• Designing PCI Cards and Drivers for Power Macintosh Computers, pages 110 through 117, documents the Driver Gestalt mechanism and some new Control requests. This technote provides clarifications and corrections on Driver Gestalt and the mechanism used to boot from a partition. In addition, the File Exchange section of this technote completely replaces the PC Exchange description in the book.
• Guide to the File System Manager contains useful background information about how FSM interacts with disk drivers; however, the specific recommendations for driver writers are covered in the File Exchange section of this technote.
• DTS Q&A OPS 22, "Notification Manager Reinitialized During Boot" is an important tidbit for disk driver developers.
• DTS Q&A DV 34, "Secondary Interrupts on the Page Fault Path" describes the dangers of using secondary interrupts in software that must service page faults. While the Q&A was written for SIM developers, its warning is also important for other page fault path software, such as disk drivers. Disk drivers must not use secondary interrupts (or, for that matter, deferred tasks) on the page fault path.
• Data Structure to Aid Security and Recovery Software, David Shayer and Marvin Carlberg, 1991
• The InterruptSafeDebug module of the DTS sample code library MoreIsBetter can be useful when tracking down nasty crashing problems in a device driver, especially those that happen early at startup time.

Obsolete

These documents, as they pertain to disk drivers, are considered obsolete. This list is provided for completeness only. You should read the recommended material instead.

• Inside Macintosh II, "The Disk Driver", page 211 through 219, documents the basic interface to a disk driver, include the kEject (7) Control request, the kSetTagBuffer (8) Control request, and the kDriveStatus (8) Status request.
• Inside Macintosh IV, "The Disk Driver", page 223 through 224, documents the kVerify (5), kFormat (6), kTrackCache (9), and kDriveIcon (21) Control requests.
• Inside Macintosh IV, "The SCSI Manager", page 292 through 293 describes the original partitioning format used on the Mac Plus and goes on to say, "Since the driver is called to install itself, it must contain code to set up its own entry in the unit table and to call its own Open routine. An example of how to do this can be obtained from Developer Technical Support." This example was part of the "SCSI Driver Developer Kit". All of the information in the kit is available elsewhere. The specific sample code referenced by the book evolved into SCSI Driver Example.
• Inside Macintosh V, "The Disk Driver", page 470 through 471, documents the kDriveIcon (21), kMediaIcon (22), and kDriveInfo (23) Control requests.
• Technote DV 2, "_AddDrive, _DrvrInstall, and _DrvrRemove" documented the AddDrive, DriverInstall, and DriverRemove system routines. This technote is now obsolete. AddDrive is documented in Inside Macintosh: Files, and DriverInstall, and DriverRemove are covered by Inside Macintosh: Devices, along with DriverInstallReserveMem. Moreover, developers of 68K drivers should use TradDriverLoaderLib to install their drivers.
• Technote DV 12, "Our Checksum Bounced" documents a misfeature of the code used by the ROM to checksum disk drivers. The technote is now obsolete. The ROM checksum behavior is described in Inside Macintosh: Devices and this technote describes the checksum algorithm itself.
• Technote DV 13, "_PBClose the Barn Door" still contains valid advice for general device driver writers, although this technote deals with this topic as it applies to disk drivers.
• Technote DV 18, "CD-ROM Notes (Most Excellent)" contains some interesting historical information about CD-ROM devices, although much of the information is now obsolete or covered elsewhere.
• Power Macintosh 9500 Computers (hardware developer note) describes many aspects of the large volume support (greater than 4 GB support) introduced with that machine. The large volume support aspects of that developer note are now obsolete. This technote discusses large volume support as it applies to disk drivers. DTS Q&As FL 07 and FL 08 discuss large volume support from the application perspective.
• The following documents were never released generally. Their developer-oriented content has been rolled into this technote.
  • "Chainable Drivers and Patches"
  • "Ruby Slipper Lite ERS" (large volume support)
  • "Bootable CD Developer Kit (Software Developer Note)"
  • "PC Exchange and Large Volume Drivers"

Checklist for Disk Driver Writers

All of the above is probably overwhelming, so here is a summary of the most important steps to take to improve the reliability and compatibility of your disk driver:

• If you do nothing else, you should support Driver Gestalt.
• You should support the partition map entry features documented in Secrets of the Partition Map. Specifically, you should ensure that your driver is checksummed, supports booting from a partition, and write your driver signature to the pmPad field.
• Your driver should support large volumes, including booting from large volumes on machines without large volume support in the ROM by means of the 'ruby' patch.
• You should follow the rules when installing and removing your driver and its drive queue elements. You should also support close to allow other developers to remove your driver cleanly.
• If your driver uses SCSI Manager 4.3 or ATA Manager, it must register itself with the manager. The documentation for each manager describes how this is done. If you're using SCSI Manager 4.3, use SCSICreateRefNumXref. If you're using ATA Manager, use kATAMgrDriveRegister.
• You should support the File Exchange interface. This will allow foreign file systems to access your disks without any skullduggery.
• You should check that your private Control and Status requests follow the rules, both with respect to Driver Gestalt and virtual memory. This is harder than you might think.
• You should support read-verify mode. This technote explains how to do it easily.
• You may want to support target mode in your ATA driver.
• You may want to support color icons. Woo hoo!

For Application Writers

The purpose of a disk driver is to support a generic interface for accessing block devices. The primary client of this interface is the File Manager, although it can be used by other programs. If you're writing a foreign file system, or just an application that needs something beyond the standard File Manager programming interface, parts of this technote may be of interest to you.

• If you need to interrogate a driver about its capabilities, you should read the section Driver Gestalt for Applications.
• If you need to read arbitrary blocks on a volume, you should read the discussion of the XIOParam block for applications, along with the accompanying hints and tips.
• If you need to read arbitrary blocks outside of a volume -- for example, the partition map, or a non-Mac OS partition -- you should investigate the File Exchange section of this technote, especially the section on using the File Exchange interface.
• If you need to verify that you have written data to the disk correctly, you should check out the read-verify mode section which describes the easiest way to do this. [Hint: Think "MoreFiles"!]
• If you need to get color icons for a drive, you can now call the disk driver to get them -- although you should probably just call Icon Services instead.

In addition, if you're writing a disk formatting utility, this technote contain invaluable information on the partition map, chaining drivers, patch partitions, and "hostile" takeovers.

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Disk Driver Basics

Mac OS communicates with attached devices through device drivers, which are software plug-ins that conform to a well-defined structure. The Device Manager is the original system component used to install, find, manage, and communicate with device drivers. It exports routines that can be called by higher level system software, and by applications. Most of these routines translate directly into requests to the underlying device driver.

In order to identify different drivers, the Device Manager assigns each installed driver a unique negative number, referred to as a driver reference number. When calling the Device Manager, clients pass a driver reference number to tell it which driver they are dealing with.

For a block device to be available to the system, it must have a disk driver. This is either in the ROM (for the built-in floppy drive), or loaded at system startup from a special partition on the disk (SCSI, ATA, and FireWire devices), or loaded from a system extension (USB and FireWire devices). In addition, a disk driver can be loaded when a device is plugged in by either an I/O family expert (ATA, USB, and FireWire), or by a special utility program (SCSI). Finally, software can install a disk driver for a virtual block device which has no obvious physical presence, such as a RAM disk or disk image. Regardless of how they are installed, all disk drivers roughly follow the same rules.

It is important to note the difference between a disk and a device. A block device is the entity which reads and writes data on a disk. A disk is the medium which actually stores the data. This distinction is unimportant for fixed disk devices (such as hard disks), but is critical for removable disk devices (such as floppy drives and removable cartridge disk devices).

Mac OS always directs block I/O to a software entity known as a drive. Each disk driver creates one or more drives and puts them in a system structure called the drive queue. Each drive queue element represents a drive, and contains both the driver reference number and the drive number. The drive number is a positive number that uniquely identifies the drive; it is assigned when the drive is added to the drive queue.

A drive does not necessarily correspond directly to a given physical device. Rather, the driver decides which drives to create for the device it controls. In some cases, there is one drive per physical device. For example, the built-in floppy disk driver creates a drive for each attached floppy disk device. However, it is also common for a driver to create multiple drives for a single device. For example, the driver for a partitioned hard disk device creates a drive for each file system partition on the disk.

When the system performs I/O to a drive, it supplies the driver reference number of the device driver and the drive number of a drive created by that driver. The Device Manager uses the driver reference number to find the device driver and call its entry point. The device driver then uses the drive number to determine which drive is the target of the I/O request.

All drive I/O is done is terms of 512-byte logical blocks. Therefore, all transfers must start at multiple of 512 bytes and be a multiple of 512 bytes long. This is regardless of the underlying device's block size.

File Manager and Drives

To allow the flexibility of storage required by the user interface (a hierarchy of folders and files), Mac OS implements another layer of abstraction, known as the File Manager, on top of the Device Manager and the drive queue.

A file system is a mechanism for storing fine-grained data (files) and meta-data (folders, Finders attributes, and so on) on a drive. The file system defines the way this data is stored and the rules for manipulating it. The File Manager includes built-in support for two file systems (HFS and HFS Plus) and a plug-in architecture (File System Manager) for others (AppleShare, DOS FAT, ProDOS , UDF, and third-party FSM plug-ins).

The File Manager exports a programming interface defined in terms of volumes, which contain directories, files, and meta-data. A volume is an instance of a file system on a drive. Each volume is uniquely identified by a negative volume reference number, which is stored, along with other data to operate the volume, in a volume control block (VCB) that is linked into the system VCB queue. The VCB also contains the drive number and the driver reference number of the drive on which the volume is mounted.

The process of making the contents of a drive available via the File Manager is called mounting a volume. When the File Manager attempts to mount a volume on a drive, it calls each of the file systems in turn to determine which one understands the logical format of the data on the disk in the drive. It then creates a VCB for that file system on that drive.

The File Manager takes requests to operate on the volume and passes them to the appropriate plug-in file system, which reduces them to basic block operations and passes them to the drive via the Device Manager (using the drive number and driver reference number stored in the VCB). As far as the file system is concerned, the drive is its own logical disk, even though it may only represent a small part of the real disk.

A drive can exist without having a volume mounted on it. This happens, for example, if the data format on the drive is incomprehensible to the installed file systems, or the volume on the drive has been unmounted. You can still access the data on a drive that has no volume mounted on it, but only via the Device Manager interface.

Terminology

In any technical document, it is very important to get your terminology straight. This is especially important when talking about disk drivers, where much of the terminology has been extended over the long, confusing history of the Mac OS block storage architecture. This technote uses the following terms throughout.

disk driver

A software plug-in that implements a hardware abstraction layer for block devices, like hard disks, floppy drives, and CD-ROM drives. In Mac OS, a disk driver must be a Device Manager driver (either a 68K driver or a native driver).

68K driver

A disk driver implemented using the traditional 68K driver architecture, as documented in Inside Macintosh: Devices. A 68K driver is commonly stored in a resource of type 'DRVR' or in a driver partition.

native driver

A disk driver implemented using the native driver model, introduced with the first generation of PCI Power Macintosh computers and documented in Designing PCI Cards and Drivers for Power Macintosh Computers. A native driver is commonly stored in a file of type 'ndrv', although native drivers have started appearing in driver partitions as well.

driver reference number

An SInt16 that uniquely identifies a Device Manager driver to the system. Driver reference numbers are not persistent -- they are assigned when the driver is added to the unit table -- but some driver reference numbers are assigned to certain well-known drivers. Driver reference numbers occupy the same "name space" as file reference numbers (which identify an open file). Driver reference numbers are always negative, while file reference numbers are always positive. Zero is an invalid driver reference number and an invalid file reference number.

unit table

A Device Manager data structure that lists the installed device drivers (both 68K and native).

block device

A block-oriented storage device.

real block device

A block device that has some obvious physical presence, such as a floppy drive or a SCSI hard disk device.

virtual block device

A block device this has no obvious physical presence, such as a RAM disk, a disk image, or a network block device.

device

Some hardware attached to the computer. In this context of this technote, this typically means a block device although, in some places, the term may be used for any type of device.

disk

The actual physical media which holds data. A disk is made up of blocks, each of which holds a fixed number of bytes (typically 512). A disk is distinct from a block device because, in the case of removable disk devices, the user can insert one of many different disks into the device.

disc

A synonym for "disk" that is only used in the context of CD or DVD discs (where the disk is actually a disc).

media

See disk.

drive

A Mac OS software construct used to represent a block storage entity. A volume is always mounted on a drive. There may be multiple drives corresponding to a single disk. Exception: some removable disk devices have been historically known as drives (for example, floppy drive, CD-ROM drive). This technote continues to use "drive" in these contexts, rather than the more cumbersome "floppy disk device." However, if the word "drive" appears unqualified, it always refers to the primary definition.

drive queue

A OS queue which contains all the drive queue elements known to the system. You can get the head of the drive queue using the routine GetDrvQHdr. See Inside Macintosh: Files for more details of the drive queue and its elements.

drive queue element

The specific data structure used to represent a drive. A drive queue element is a structure of type DrvQEl allocated in the system heap and placed in the drive queue.

drive number

An SInt16 which uniquely identifies a drive. Drive numbers are not persistent; they are assigned when the drive is added to the drive queue. Drive numbers occupy the same "name space" as volume reference numbers. Drive numbers are always positive, while volume reference numbers are always negative.

partition

A disk may be divided into a set of contiguous blocks, each known as a partition. Partitions are typically either file system partitions (which hold file system data) or meta-data partitions (which hold information about the disk, such as the partition map or the disk's device driver). Not all disks are partitioned, although a disk must be partitioned to support booting (except for floppy disks, because the driver for the built-in floppy disk drive is in the ROM).

partition map

A data structure, typically at the beginning of the disk, which describes the partitions on the disk. Most Mac OS disks are partitioned using the Apple partition map format, described in Secrets of the Partition Map.

partition map entry

The Apple partition map describes each partition on the disk using a partition map entry data structure (of type Partition).

startup partition

The partition which the user has designated as the one from which they prefer to boot the system, or the partition from which the system booted.

driver partition

A partition which contains a disk driver.

file system partition

A partition which contains file system data.

meta-data partition

A partition which holds information about the disk, such as the partition map or the disk's device driver.

partition-based driver

A driver that is loaded from a partition.

file system-based driver

A driver that is loaded from a file in the file system, typically in the Extensions folder.

disk-based driver

Either a partition-based driver or a file system-based driver. This term is ambiguous and to be avoided.

ghost partitioning

A system used on non-512 byte block devices where partition map entries appear at both 512-byte boundaries and device block boundaries so that they can be seen by software using either physical or device blocks.

I/O family

A component of the Mac OS I/O subsystem that is responsible for a particular category of devices. A driver can work within multiple I/O families. Each family requires certain attributes of the driver (for example, how it is packaged and the programming interface it provides to upper layer software) and provides services for the driver. For example, a FireWire disk driver must be packaged as a native driver which responds to the standard disk driver programming interface, and FireWire provides services to the disk driver, such as SBP-2 utility routines.

I/O family expert

A component of an I/O family that seeks out devices of a particular type and registers them with the I/O family.

volume

A File Manager software construct that represents a single, user-visible storage device. Each volume appears as a icon on the desktop. Each volume is mounted on a drive, so if the disk has multiple file system partitions it will also have multiple drives and hence multiple volumes.

volume reference number

An SInt16 which uniquely identifies a volume. Volume reference numbers are not persistent; they are assigned when the volume is mounted. Volume reference numbers occupy the same 'name space' as drive numbers. Drive numbers are always positive, while volume reference numbers are always negative.

refNum

This contraction of "reference number" is ambiguous and is not used in this document. In other documents, it commonly means either a driver reference number or a file reference number, depending on context.

vRefNum

A contraction of volume reference number.

logical blocks

The block numbering scheme used to access blocks on a drive. Each logical block contains 512 bytes and the first block accessible through the drive is block 0. See Block Translation for details.

physical blocks

The block numbering scheme used to access blocks on a disk. You can derive a physical block number from a logical block number by adding to it the start block number of the partition. If the disk is not partitioned, logical blocks and physical blocks are identical. Each physical block contains 512 bytes. See Block Translation for details.

device blocks

The actual block numbering scheme used by the device hardware to access data on the disk. Device blocks are not necessarily 512 bytes big, and the device driver is responsible for blocking and deblocking to present the illusion of 512-byte physical blocks to the system. See Block Translation for details.

blocks

When used without qualification in this technote, blocks means logical blocks.

sectors

Depending on context, this can either mean device blocks (for a floppy drive), physical blocks (for a hard disk device), or logical blocks (in a volume format specification). To avoid confusion, this technote avoids the term "sector" in favor of its more specific synonyms.

chaining driver

A driver loaded from a partition which performs some action and then loads the next driver in the driver chain. The most common chaining driver is Apple's patch driver.

driver chain

A sequence of drivers, each in its own driver partition, that can all be loaded for a particular expansion bus type (for example, SCSI or ATA). Each driver chain consists of one or more chaining drivers and a real driver for the disk. A disk may contain more than one driver chain if it can be accessed through more than one expansion bus type.

patch driver

A chaining driver which applies the patches from a patch partition and then chains to the next driver.

patch partition

A meta-data partition containing patches that must be applied to the system before it can boot. The patches in the patch partition are applied by the patch driver before it chains to the real disk driver.

target mode

PowerBook computers can be placed in target mode, where the PowerBook's internal hard disk device is accessible as a hard disk device to other computers on an expansion bus (typically SCSI).

SCSI disk mode

See target mode.

request

When the Device Manager calls a driver entry point (Open, Close, Prime, Control, or Status for a 68K driver, DoDriverIO for native drivers), it passes the address of a parameter block which describes the requested operation. This is known as a request. A request is different from a simple function call in that the driver may return from this initial call without completing the request. Specifically, for queued requests, the request is not complete until the driver explicitly tells the system so (by calling IODone for 68K drivers, or by calling IOCommandIsComplete for native drivers).

queued request

Synchronous and asynchronous requests are collectively known as queued requests. This is because they are queued in the driver's queue (on the dCtlQHdr) and the driver is marked as busy while the request is being processed.

immediate request

Immediate requests are distinct from queued requests in that they are not placed in the driver's queue and do not mark the driver as busy.

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Driver Gestalt

All disk drivers should support Driver Gestalt. Driver Gestalt is a mechanism whereby the system can query your driver to determine whether it supports advanced driver features. In many ways it is similar to the Mac OS Gestalt Manager, except that the system is querying your driver, not the other way around.

Your driver should support Driver Gestalt. If you don't support Driver Gestalt, the system is in the dark as to which advanced driver features your driver supports.

Driver Gestalt Reference

The basic reference for Driver Gestalt is Designing PCI Cards and Drivers for Power Macintosh Computers, specifically the "Driver Gestalt" section starting on page 106. However, Driver Gestalt is useful even on non-PCI computers. Your driver must support Driver Gestalt regardless of what computer or OS version it is running on.

Designing PCI Cards and Drivers for Power Macintosh Computers does not document all of the selectors associated with Driver Gestalt. The only official, up-to-date list of Driver Gestalt selectors is the "DriverGestalt.h" header file, provided as part of Universal Interfaces. When Apple defines a new Driver Gestalt selector, we add the selector to "DriverGestalt.h", along with comments that describe how to implement it.

In the event of a conflict between the written documentation and "DriverGestalt.h", "DriverGestalt.h" is correct and the written documentation is wrong. For example, Designing PCI Cards and Drivers for Power Macintosh Computers describes the response of the 'purg' selector as a Boolean (page 111), whereas "DriverGestalt.h" correctly describes the response to be of type DriverGestaltPurgeResponse.

Driver Gestalt Guarantees

By saying that it supports Driver Gestalt, your driver guarantees certain things to the system, including:

1. Your driver will return controlErr in response to a Control request with an unrecognized csCode.
2. Your driver will return statusErr in response to a Status request with an unrecognized csCode.
3. Your driver will return controlErr in response to a Driver Configure request with an unrecognized selector.
4. Your driver will return statusErr in response to a Driver Gestalt request with an unrecognized selector.
5. Your driver will not use any csCodes below 128 for private Control or Status requests.

Items 3 and 4 in the list above are not documented clearly in Designing PCI Cards and Drivers for Power Macintosh Computers, although they are implemented by all Apple drivers and are clearly shown in the various Driver Gestalt samples. This technote serves to officially document these two additional requirements.

Driver Gestalt for Applications

Probably the best way to understand how to issue Driver Gestalt queries from an application is to look at some sample code. "Driver Gestalt Demo" is a simple sample that shows how to issue a few queries. "DriverGestaltExplorer" is a more comprehensive sample, which is also useful as a simple test and investigation tool. Both samples are available as DTS sample code.

Summary of Driver Gestalt

All disk drivers should support Driver Gestalt.

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Secrets of the Partition Map

A number of features have been added to the Apple partition map since it was documented in Inside Macintosh: Devices. This section describes those features in detail.

Partition Field Relevance

The description of the Partition data type in Inside Macintosh: Devices does not explicitly call out that some fields of the data structure are only relevant for driver partitions (those whose partition name contains "Apple" and "Driver"). Specifically, the fields from pmLgBootStart through to pmProcessor are only relevant for driver partitions. Non-driver partitions should set these fields to zero.

pmParType Possibilities

Inside Macintosh: Devices documents the well known values for the pmParType field of the partition map entry, namely "Apple_partition_map", "Apple_Driver", "Apple_Driver43", "Apple_MFS", "Apple_HFS", "Apple_Unix_SVR2", "Apple_PRODOS", "Apple_Free", and "Apple_Scratch". This technote describes a number of additional partition types.

• "Apple_Driver_ATA" -- Holds the device driver for an ATA device.
• "Apple_Driver_ATAPI" -- Holds the device driver for an ATAPI device. When it discovers a device on an ATA bus, the ATA Manager identifies whether a device is ATA or ATAPI and automatically loads the corresponding driver.
• "Apple_Driver43_CD" -- A SCSI CD-ROM driver suitable for booting.
• "Apple_FWDriver" -- Holds a FireWire driver for the device. See Loading FireWire Drivers for details.
• "Apple_Void" -- A dummy partition map entry, used to pad out a partition map to ensure the correct alignment of partition map entries in a bootable CD-ROM.
• "Apple_Patches" -- Holds a patch partition. The patch partition architecture is described in Chaining Drivers and Patch Partitions.

pmPartStatus Revealed

Inside Macintosh: Devices says that the pmPartStatus field of the Partition data structure is only used by A/UX, bits 0 through 7 having a defined meaning and all others being reserved. This is no longer true.

The following flags are defined in pmPartStatus field of the Partition structure. All bits not defined here are reserved (you should initialize them to 0 and ignore their value).

enum {
    kPartitionAUXIsValid= 0x00000001,
    kPartitionAUXIsAllocated  = 0x00000002,
    kPartitionAUXIsInUse= 0x00000004,
    kPartitionAUXIsBootValid  = 0x00000008,
    kPartitionAUXIsReadable   = 0x00000010,
    kPartitionAUXIsWriteable  = 0x00000020,
    kPartitionAUXIsBootCodePositionIndependent  = 0x00000040,
 
    kPartitionIsWriteable     = 0x00000020,
    kPartitionIsMountedAtStartup = 0x40000000,
    kPartitionIsStartup = 0x80000000,
 
    kPartitionIsChainCompatible  = 0x00000100,
    kPartitionIsRealDeviceDriver = 0x00000200,
    kPartitionCanChainToNext  = 0x00000400,
};

Bits 0 through 4 and 6 are still defined as documented in Inside Macintosh: Devices. A Mac OS formatting utility should always set these bit to 1 for file system partitions and clear them for other partition types.

The second group of bits is used by Apple Mac OS disk drivers to hold information about file system partitions.

kPartitionIsWriteable

This bit indicates whether the partition is writeable (1) or write-protected (0). If the bit is clear and your driver creates a drive queue element to represent this partition, it should mark the drive queue element as write-protected. Note that mask has the same value (and the same semantics) as kPartitionAUXIsWriteable.

kPartitionIsMountedAtStartup

This bit indicates whether the partition is mounted at system startup (1) or not (0). If your driver would otherwise create a drive queue element to represent this partition at system startup and this bit is clear, it should not create the drive.

kPartitionIsStartup

This bit indicates whether this is the startup partition (1) or not (0). This bit must be set for at most one partition. See A Partition of Your Imagination below.

The third group of bits provides information about driver partitions. You may need to read Chaining Drivers and Patch Partitions to understand these descriptions.

kPartitionIsChainCompatible

The driver in this partition supports being loaded by a chaining driver.

kPartitionIsRealDeviceDriver

This partition contains a driver that actually knows how to drive the device. Contrast this with the patch driver, which is chain compatible, but which can only load patches and then chain to the next driver; it does not actually contain a disk driver.

kPartitionCanChainToNext

This partition contains a driver that can chain to another driver. Typically, all drivers in the chain must have this bit set, except the last one where it is clear.

Partition Attributes

There are a number of Control and Status requests that modify the attributes of a partition. A disk driver must support these requests as described below. A formatting application can use these requests to modify partition attributes.

Setting the Startup Partition

In response to this request, your disk driver must set the partition described by ioVRefNum and csParam[0..3] as the startup partition. Typically this involves setting kPartitionIsStartup in pmPartStatus, which in turn causes your disk driver to place the drive queue element for this partition first in the drive queue at system startup.

Determining Whether a Partition is the Startup Partition

In response to this request, your disk driver must set csParam[0] to indicate whether the partition described by ioVRefNum and csParam[0..3] is the startup partition. Typically this involves testing kPartitionIsStartup in pmPartStatus.

The request returns the status that is currently recorded in the partition map, not whether the system actually started from this partition.

Specifying That a Partition Should Be Mounted at Startup

In response to this request, your disk driver must set the partition described by ioVRefNum and csParam[0..3] to be mounted at startup. Typically this involves setting kPartitionIsMountedAtStartup in pmPartStatus, which in turn causes your disk driver to place a drive queue element for this partition in the drive queue at system startup.

This request modifies the partition map, and hence only takes effect the next time the system is started. It does not affect the state of any volume currently mounted on the partition.

Specifying That a Partition Should Not Be Mounted at Startup

In response to this request, your disk driver must set the partition described by ioVRefNum and csParam[0..3] to not be mounted at startup. Typically this involves clearing kPartitionIsMountedAtStartup in pmPartStatus, which in turn causes your disk driver to not place a drive queue element for this partition in the drive queue at system startup.

This request modifies the partition map and hence only takes effect the next time the system is started. It does not affect the state of any volume currently mounted on the partition.

Determining Whether a Partition is to be Mounted

In response to this request, your disk driver must set csParam[0] to indicate whether the partition described by ioVRefNum and csParam[0..3] is to be mounted at system startup. Typically this involves testing kPartitionIsMountedAtStartup in pmPartStatus.

The request returns the status that is currently recorded in the partition map, not whether the partition was actually mounted at startup.

Mounting a Partition Immediately

In response to this request, your disk driver must create a drive queue element for the partition described by ioVRefNum and csParam[0..3] (if it doesn't already have one) and post a "disk inserted" event for it. It must do this regardless of the state of the kPartitionIsMountedAtStartup bit in the partition's pmPartStatus; however, the kPartitionIsWriteable bit still controls whether the drive is writeable.

If there is already a volume mounted on the partition, the system will ignore the "extra disk inserted" event this request generates.

Locking a Partition

In response to this request, your disk driver must lock the partition described by ioVRefNum and csParam[0..3]. Typically this involves:

• clearing kPartitionIsWriteable in pmPartStatus, which in turn causes your disk driver to create a read-only drive queue element for this partition at system startup, and
• making the drive queue element associated with this partition read-only. A read-only drive queue element has bit 7 of the writeProt field of the drive queue element set, as described in Inside Macintosh: Files, page 2-85.

Unlocking a Partition

In response to this request, your disk driver must unlock the partition described by ioVRefNum and csParam[0..3]. Typically this involves:

• setting kPartitionIsWriteable in pmPartStatus, which in turn causes your disk driver to create a read/write drive queue element for this partition at system startup, and
• making the drive queue element associated with this partition read/write.

Determining Whether a Partition is Locked

In response to this request, your disk driver must set csParam[0] to indicate whether the partition described by ioVRefNum and csParam[0..3] is locked. Typically this involves testing kPartitionIsWriteable in pmPartStatus.

The request returns the status that is currently recorded in the partition map, not whether the partition was actually locked at startup. You can determine whether a drive is currently write-protected by looking at bit 7 of the writeProt field of the drive queue element, as described in Inside Macintosh: Files, page 2-85.

pmPad Pearls

A previously undocumented feature of the Partition structure is the use of the pmPad field. The first four bytes of this field is a driver signature, a Mac OS four- character code that uniquely identifies the driver. Developers must fill out this field with either a registered creator code (which is strongly recommended) or zero. Drivers that use a registered creator code in this driver signature field may then use the remainder of pmPad to hold driver-specific configuration parameters.

Apple currently uses the following driver signatures:

enum {
    kPatchDriverSignature   = 'ptDR',
    kSCSIDriverSignature    = 0x00010600,
    kATADriverSignature     = 'wiki',
    kSCSICDDriverSignature  = 'CDvr',
    kATAPIDriverSignature   = 'ATPI',
    kDriveSetupHFSSignature = 'DSU1'
};

The values have the following meaning:

kPatchDriverSignature

The Apple patch driver.

kSCSIDriverSignature

The Apple SCSI hard disk driver. [The significance of this value has been lost in the mists of time.]

kATADriverSignature

The Apple ATA hard disk driver.

kSCSICDDriverSignature

The Apple SCSI CD-ROM driver.

kATAPIDriverSignature

The Apple ATAPI CD-ROM driver.

kDriveSetupHFSSignature

Drive Setup sets the first four bytes of the pmPad field of "Apple_HFS" partitions to this value. While this is not, in the strictest sense, a driver signature, it is documented here for completeness.

Remember that your disk driver should use its own driver signature; do not use these values for your own driver.

New Driver Types

Inside Macintosh: Devices describes how a Mac OS driver is tagged by having ddType set to 1 in the driver descriptor map (DDM). There is a constant for this, sbMac, defined in "SCSI.h". However, there are other useful constants for this field.

enum {
    kDriverTypeMacSCSI = 0x0001,
    kDriverTypeMacATA  = 0x0701,
    kDriverTypeMacSCSIChained   = 0xFFFF,
    kDriverTypeMacATAChained    = 0xF8FF
};

The following constants are defined for the ddType field of the DDM:

kDriverTypeMacSCSI

This is a Mac OS SCSI driver, equivalent to sbMac. Typically this is only used for the first driver (the patch driver) in a SCSI driver chain.

kDriverTypeMacATA

This is a Mac OS ATA driver. Typically this is only used for the first driver (the patch driver) in an ATA driver chain.

kDriverTypeMacSCSIChained

This is a chained Mac OS SCSI driver. This is used for the second and subsequent drivers in a driver chain.

kDriverTypeMacATAChained

This is a chained Mac OS ATA driver. This is used for the second and subsequent drivers in a driver chain.

The driver type for a chained driver is always the two's complement of the driver type for the patch driver. For more information about this relationship, see Chaining Drivers and Patch Partitions.

Driver Checksums

Inside Macintosh, Volume V (page 580) contains an assembly language description of the checksum algorithm used for the pmBootCksum field of the partition map, but this algorithm was somehow dropped from Inside Macintosh: Devices. As it is now quite difficult to obtain copies of Inside Macintosh, Volume V, the algorithm is included below.

; Inputs:
;   a0.l -> pointer to driver code
;   d1.w -> length of driver code in bytes
; Outputs:
;   d0.w -> driver checksum
 
DoCksum
  moveq.l     #0,d0 ; initialize sum register
  moveq.l     #0,d7 ; zero extended byte
  bra.s CkDecr; handle 0 bytes
CkLoop
  move.b(a0)+,d7    ; get a byte
  add.w d7,d0 ; add to checksum
  rol.w #1,d0 ; and rotate
CkDecr
  dbra  d1,CkLoop   ; next byte
  tst.w d0 ; convert a checksum of 0
  bne.s @1 ; into $FFFF
  subq.w#1,d0 ;
@1

The following is a C equivalent.

static UInt32 ChecksumDriver(void *start, UInt16 bytesToSum)
{
    UInt8  *cursor;
    UInt16 result;

    cursor = (UInt8 *) start;
    result = 0;

    while ( bytesToSum != 0 ) {
  result = result + *cursor;
  result = ((result <<  1) & 0x0FFFE) |
  ((result >> 15) & 0x00001);
  cursor += 1;
  bytesToSum -= 1;
    }
    if (result == 0) {
  result = 0x0FFFF;
    }
    return result;
}

One minor mystery of the pmBootCksum field is that the field is 32 bits wide but the checksum algorithm only calculates a 16-bit value. The checksum is always stored in the least significant 16 bits of pmBootCksum and the most significant bits are always set to zero.

Inside Macintosh, Volume V also states that driver checksumming is only done for if the first four bytes of the driver's partition map entry pmPartName field is "Maci". This is only true for SCSI disk drivers. Other, partition-based disk drivers are always checksummed.

The above algorithm is known as the 16-bit driver checksum algorithm. This is because the ROM decrements and tests bytesToSum using a DBRA instruction (which effectively makes bytesToSum a UInt16), so only the first bytesToSum modulo 64 K bytes of the driver are checksummed. This is not a problem if your driver is smaller than 64 K bytes. If your driver is larger, you must be careful for two reasons.

1. The code you use to calculate pmBootCksum must mimic the incorrect behavior and only checksum your driver up to the driver size modulo 64 K.
2. You may want to include your own checksum in the driver to ensure that the driver code is intact.

static UInt16 ATALoadDoCksum(void *start, UInt32 bytesToSum)
{
    UInt8  *startAsBytes;
    UInt32 result;
    UInt32 i;

    startAsBytes = (UInt8 *) start;
    result = 0;
 
    for (i = 0; i < bytesToSum; i++) {
  result += startAsBytes[i];
  result <<= 1;
  result |= (result & 0x00010000) ? 1 : 0;
    }
    return (UInt16) result;
}

The key difference is that bytesToSum is now expressed as a 32-bit quantity, and the algorithm correctly checksums bytes beyond 64 KB. Further, the 16-bit algorithm never returns a checksum of 0 (it is mapped to $FFFF), while the 32-bit algorithm can return a checksum of 0.

Your formatting utility must set pmBootCksum appropriately, depending on which version of ATA Manager is loading your driver. Furthermore, the ATA driver loader mechanism is updated during the system startup process so that on machines with the old checksum algorithm in ROM, your driver will need a different checksum depending on whether it is loaded at start time or after system startup.

Overall, the best solution to this driver checksum conundrum is:

• make your driver's size less than 64 KB (if necessary, use a boot strap driver to load your main driver), and
• if your driver checksums to 0, add pad bytes until it doesn't.

A Partition of Your Imagination

The original Mac Plus SCSI implementation did not allow the user to specify a startup partition. Obviously this is desired feature, and disk driver developers came up with a number of solutions for this problem. Over the years, Apple has introduced various stages of OS support for booting from a partition.

Developer-Only Solutions

Prior to Apple providing a solution, developers were responsible for engineering their own. Developers quickly noticed that, all things being equal, the Macintosh tends to boot from the first bootable drive in the drive queue. Therefore, disk driver writers arranged to add the startup partition's drive queue element to the drive queue before the non-boot partitions' element. The disk driver's formatting utility provided the user interface for specifying the boot partition.

This technique was relatively effective and stimulated user demand for a reliable mechanism for booting from a partition.

Partition Attribute Support

Eventually, Apple codified this approach and provided support for it in the Startup Disk control panel. The codification came in the form of the kPartitionIsStartup bit in the pmPartStatus field of the partition map, along with a driver Control request, kSetStartupPartition, which allows the Startup Disk control panel to instruct the driver to set that bit.

This standardized the previous non-standard behavior, although it still is not a perfect solution because of variances in the way the ROM startup code chooses a drive from which to start up.

SCSI Manager 4.3

Apple made further refinements to this solution with the introduction of SCSI Manager 4.3. SCSI Manager 4.3 presented new problems to the startup code because it allows for multiple SCSI buses, and it provides full support for SCSI LUNs. So, when SCSI Manager 4.3 was introduced, Apple also introduced a new technique for finding the startup partition, the kdgBoot Driver Gestalt selector.

When the user chooses a drive in the Startup Disk control panel, Startup Disk sends the kdgBoot Driver Gestalt selector to the disk driver controlling that drive. Startup Disk then records the response in PRAM. When the Macintosh boots, it iterates through the drive queue, sending a kdgBoot request to each drive. When it finds a drive with a value matching the value in PRAM, it knows that this is the correct startup drive.

The kdgBoot Driver Gestalt selector is documented in Designing PCI Cards and Drivers for Power Macintosh Computers, page 113. This documentation is accurate for SCSI drivers. For ATA drivers, the DriverGestaltBootResponse response fields should be set as follows.

extDev

The ATA bus number of the device.

partition

The partition number on the bootable partition on the device. As described below, the format of this field is internal to your disk driver.

SIMSlot

ATA devices must set this to kDriverGestaltBootATASIMSlot ($20). [This constant is not currently in Universal Interfaces, Radar ID 2314693.]

SIMsRSRC

If your driver supports ATA 0/1, you must put 0 or 1 in this field to indicate the number of the device on the ATA bus. If your driver does not support ATA 0/1, you must set this to zero. See ATA 0/1 Software Developers Guide for more details on ATA 0/1 support.

ROM-in-RAM (NewWorld)

The ROM-in-RAM architecture, introduced with the iMac, presents new challenges for the startup device selection process. On a ROM-in-RAM machine, Open Firmware is responsible for loading the Mac OS ROM file off the startup partition, and hence Open Firmware must define the startup partition well before Mac OS starts to execute. When the Mac OS ROM starts, it continues booting from the startup partition chosen by Open Firmware to avoid the potential user confusion of loading the Mac OS ROM from one disk and the system software from another.

Open Firmware synthesizes the traditional Macintosh startup process, including:

• Startup drive selection algorithm -- Open Firmware implements the traditional startup drive selection algorithm. It turns out that this algorithm is very complex, although the gist of it is:
  1. if a "snag" key is held down, try booting from the corresponding device,
  2. try booting from the default drive (if any),
  3. then try booting from other drives.
• CODS -- Holding down command-option-delete-shift (CODS) prevents the Open Firmware from booting from the default drive.
• C for CD-ROM -- Holding down the C key forces Open Firmware to boot from the CD-ROM device. This was previously implemented by the "snag" patch but is implemented by Open Firmware in ROM-in-RAM computers.
• Flashing question mark -- If no startup device is available, Open Firmware displays the traditional "flashing question mark" icon (although, in deference to the fact that ROM-in-RAM computers do not have floppy drives, it flashes the question mark inside a folder icon instead of a floppy disk icon).

On ROM-in-RAM computers, the selected default startup device is held in an Open Firmware configuration variable boot-device. This configuration variable holds an Open Firmware path to the default startup device. The Startup Disk control panel generates a path based on the disk driver's response to various Driver Gestalt queries.

It is impossible for Open Firmware to completely mimic the startup drive selection algorithm when it comes to selecting a startup partition. When booting from a partition, boot-device contains the Open Firmware partition number of the startup partition. Unfortunately, there is no reliable way to get this from a disk driver with commonly implemented Driver Gestalt queries.

The partition field enables the selection of a single partition on a multiply partitioned device as the boot device. It is not interpreted by the ROM or the Startup Disk 'cdev' [sic], so the driver can choose a meaning and a value for this field.

It turns out that different disk drivers use different values for the partition field. Apple disk drivers set this to be the block number of the partition map entry for the partition, but some third-party drivers use other techniques, such as recording 1 for the first HFS partition, 2 for the second HFS partition, and so on. The upshot of this is that Startup Disk is unable to use this field reliably to set the partition number in boot-device.

Prior to Mac OS 9.0, the Startup Disk control panel used tricky heuristics to allow booting from a partition with Apple disk drivers as a temporary measure to solving this problem. The long-term solution; however, is for disk drivers to support a set of new Driver Gestalt queries, which return exactly the information Startup Disk needs to set boot-device. The required Driver Gestalt selectors (kdgDeviceReference, kdgNameRegistryEntry, kdgOpenFirmwareBootSupport, and kdgOpenFirmwareBootingSupport) are described in "DriverGestalt.h" in Universal Interfaces 3.3.

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Non-512 Byte Block Devices

The original Mac OS disk driver architecture assumed that all block devices would use 512-byte blocks. Supporting block devices with a different block size is relatively simple, although it gets more complicated if you want to boot from such a device. Non-512 byte block device support is most important for CD-ROM drivers, which use a 2-KB block size.

Just the Basics

The basic rule for supporting non-512 block devices on Mac OS is that the disk driver is responsible for blocking and deblocking all I/O requests to a drive. This discussion assumes that the device block size is an integer multiple of 512, although similar algorithms work for weird device block sizes.

Block Translation

The File Manager makes an I/O request in terms of 512-byte logical block numbers on a particular drive. The disk driver is responsible for translating the logical block number of the request to an actual block number on the drive. If the disk is partitioned, the first step of this translation is to add the offset of the partition to the logical block number; this generates the physical block number. If the device uses 512-byte blocks, the physical block number is the actual block number of the data on the disk. If the device uses non-512 byte blocks, the disk driver must do a further translation, converting the physical block number to a device block number by dividing the physical block number by the number of 512-byte blocks in each physical block.

In addition, the disk driver must block/deblock the request. If the physical block number, or the number of blocks to transfer, is not evenly divisible by the device block size, the disk driver must transfer partial blocks to and from the disk.

The following diagrams shows the entire translation process for two partitions on a 2 KB block device. All numbers on the diagram are in the units labeled in the left column. For example, partition 1 is a 50 MB partition which extends from 0 to 100 mega logical blocks (512-byte blocks), 40 to 140 mega physical blocks (also 512-byte blocks), and 10 to 35 mega device blocks (2 KB byte blocks).

Implementation Notes

A disk driver typically deblocks a request by breaking it into three components. The leading component consists of all the requested physical blocks up to the first device block boundary. The leading component is empty if the requested physical blocks start on a device block boundary.

The main component consists of all the requested physical blocks which are fully encompassed by device blocks. The main component may be empty if the transfer is short. The main component is transferred directly from between the client buffer and the disk.

Finally, the trailing component consists of all the requested physical blocks of the transfer which fall after the last block of the main component. The trailing component is empty if the physical block number plus the number of physical blocks to transfer falls on a device block boundary.

Because you can't transfer a sub-block size request, the leading and trailing components must be transferred through a temporary buffer. You should allocate this temporary buffer when your driver is opened. As the leading and trailing components are always less than one device block (otherwise they would be part of the main component), the temporary buffer need only be as big as a device block. If your device driver is single threaded, you need only allocate a single temporary buffer. If your driver is multi-threaded, you must allocate as many temporary buffers as you allow threads of execution within your driver, or internally serialize the use of the temporary buffer.

The leading and trailing components are read by transferring the device block to the temporary buffer and then copying the appropriate data out of the temporary buffer to the client buffer. The leading and trailing components are written by first reading the current contents of the device to the temporary buffer, then copying the new data from the client buffer to the temporary buffer, then writing the temporary buffer to the device.

The following illustration shows how misaligned read is transferred to the client buffer:

Performance Considerations

The above algorithm is obviously inefficient if transfers are misaligned, that is, if the leading and trailing components are not empty. Misaligned writes are even more expensive than misaligned reads because the disk driver must do an extra I/O to pre-fill the temporary buffer with the existing contents of device block. Worse yet, a misaligned write that has both leading and trailing components takes five I/O operations (read leading, write leading, write main, read leading, write leading).

There are a number of ways to avoid misaligned transfers:

• Your formatting utility should always start partitions (especially file system partitions) on device block boundaries.
• File system clients can issue a Driver Gestalt kdgMediaInfo request to determine the device block size and ensure that transfers are aligned. This is particularly important for write requests.
• As a rule, volume formats should use the above technique to ensure that their allocation blocks are correctly aligned. At a minimum, volume formats should align allocation blocks on 2 KB boundaries to accommodate the most common cases, namely CD-ROM, DVD-ROM/RAM, and magneto-optical devices.

It is strongly recommended that your disk driver cache at least one device block. Many Mac OS programs will transfer data in sequential 512-byte chunks. By caching a single device block, your driver can radically reduce the average time taken to service these requests.

Booting From Non-512 Byte Block Devices

This section is not yet finished and has been omitted in the interests of shipping an initial version of the technote. A future revision of this technote will cover booting from a non-512 byte block device. If you are interested in this topic, please email DTS and ask for a prerelease draft of this section.

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Large Volume Support

When Mac OS originally shipped, it supported volume sizes up to 2 GB. This limit was shared by a number of system components, including the File Manager and disk drivers. Large volume support was introduced in two phases.

1. System 7.5 introduced support for volumes larger than 2 GB, up to a size of 4 GB. The semantics of two programming interfaces were changed to accomplish this.
   • PBHGetVInfo does not return the true size of the volumes greater than 2 GB; the volume size and free space are always clipped to 2 GB or less.
   • The dCtlPosition field of the Device Control Entry (DCE) was redefined as an unsigned quantity.
2. System 7.5.2 introduced support for volumes larger than 4 GB, up to a size of a 2 TB. This required two new programming interfaces.
   • PBXGetVolInfo returns the volume size and free space as a 64-bit quantity.
   • The I/O parameter block passed to disk drivers was extended to include a 64-bit field, ioWPosOffset, which supplants dCtlPosition.

The changes to the File Manager programming interfaces are not relevant to this technote; they are documented in DTS Q&A FL 08, "Determining Volume Size." This section describes the changes to the disk driver interface.

Large Volume Interfaces

Supporting volumes between 2 GB and 4 GB was simply a matter of redefining the dCtlPosition field of the DCE and the ioPosOffset field of the IOParam structure to be unsigned longs (UInt32).

To support volumes larger than 4 GB, a new extended I/O parameter block (XIOParam) structure was defined. The original and extended I/O parameter blocks are distinguished by the kUseWidePositioning bit of the ioPosMode field (clear for original, set for extended).

The C definition of the extended I/O parameter block is given below. The key difference is the addition of the ioWPosOffset field, a signed 64-bit quantity which contains the offset of the request.

struct XIOParam {
    QElemPtr   qLink;
    shortqType;
    shortioTrap;
    Ptr  ioCmdAddr;
    IOCompletionUPP     ioCompletion;
    OSErrioResult;
    StringPtr  ioNamePtr;
    shortioVRefNum;
    shortioRefNum;
    SInt8ioVersNum;
    SInt8ioPermssn;
    Ptr  ioMisc;
    Ptr  ioBuffer;
    long ioReqCount;
    long ioActCount;
    shortioPosMode;
    wide ioWPosOffset;
};
typedef struct XIOParam XIOParam;
typedef XIOParam *XIOParamPtr;

For software making extended I/O requests, the fields are defined as follows:

qLink
qType
ioTrap
ioCmdAddr

Used internally by the Device Manager.

ioCompletion

For asynchronous requests, you must either set this field to zero or set it to a universal procedure pointer for your completion routine. For synchronous requests, this field is ignored.

ioResult

On completion this field contains the result of the request, which is either noErr (0) or a negative error code. The Device Manager guarantees that this field will be set to ioInProgress (1) until the request is complete.

ioNamePtr

Ignored for _Read and _Write requests.

ioVRefNum

You must set this field to the drive number of the drive you wish to read or write.

ioRefNum

You must set this field to the driver reference number of the device driver controlling the drive you wish to read or write.

ioVersNum
ioPermssn
ioMisc

Ignored for _Read and _Write requests.

ioBuffer

You must set this to point to a data buffer from which data is written, or to which data is read.

ioReqCount

You must set this field to the number of bytes you wish to read or write. For disk driver requests, this must be a multiple of 512 bytes.

ioActCount

On completion this field contains the number of bytes of data that were actually transferred.

ioPosMode

You must set this field to kUseWidePositioning to indicate that this is a wide request. All wide requests use a positioning mode of fsFromStart. You must not specify any other positioning mode (fsAtMark, fsFromLEOF, or fsFromMark). You may also specify rdVerifyMask for read-verify mode, noCacheMask to request that the data not be placed in the cache, or pleaseCacheMask to request that data be placed in the cache.

ioWPosOffset

You must set this field to the offset (in bytes) from the beginning of the disk where the transfer should begin. For disk driver requests, this must be a multiple of 512 bytes.

For disk drivers servicing an extended I/O request, the fields are defined as follows:

qLink
qType

Used internally by the Device Manager.

ioTrap

Your driver must test bit 0 of this field to determine whether the request is a _Read (bit 0 clear) or a _Write (bit 0 set). It must also test noQueueBit (bit 9) to determine whether the request is immediate (bit 9 set) or not. If your driver does not support immediate requests, it must fail the request with a paramErr. Your driver must not test asyncTrpBit (bit 10) to determine whether the request is synchronous or asynchronous. Instead, it should handle all requests as if they were made asynchronously. See Technote 1067 Traditional Device Drivers: Sync or Swim for details.

ioCmdAddr

Used internally by the Device Manager.

ioCompletion

The Device Manager IODone routine will do the right thing with this field. Your driver should ignore this field and handle all requests as if they were made asynchronously. See Technote 1067 Traditional Device Drivers: Sync or Swim for details.

ioResult

Your driver must not read or write this field. Your driver sets this field implicitly when it calls IODone. When your driver has finished a queued request, it should call IODone to signal that the request is complete. IODone performs a number of actions, one of which is to set this field to the error status you passed to the routine in register D0. Your driver must pass a non-positive error status to IODone.

ioNamePtr

Your driver must ignore this field.

ioVRefNum

Your driver must use this field to determine which drive is the target of the request. If your driver does not control a drive with this drive number, it must complete the request with nsDrvErr.

ioRefNum

Your driver may look at this field to determine the driver reference number of the request. This may be useful if the same code is used for multiple device drivers (see Code Sharing).

ioVersNum
ioPermssn
ioMisc

Your driver must ignore these fields.

ioBuffer

Your driver must transfer data to or from the buffer pointed to by this field.

ioReqCount

Your driver must attempt to transfer the number of bytes specified in this field. Your driver may fail a request (with paramErr) if this is not a multiple of 512 bytes.

ioActCount

Before completing the request, your driver must set this field to the number of bytes that were actually transferred.

ioPosMode

Your driver must test the kUseWidePositioning bit to determine whether this is a wide request, as described in the next section. If it is a wide request, your driver must ignore the bottom 2 bits of this field (that is, fsFromStart, fsAtMark, fsFromLEOF, and fsFromMark) and use ioWPosOffset to determine the offset into the drive for the transfer. Your driver may choose to honor the rdVerifyMask, noCacheMask, and pleaseCacheMask in the traditional way.

ioWPosOffset

Your driver must transfer data from this offset (in bytes) into the drive. Your driver may fail a request (with paramErr) if this is not a multiple of 512 bytes. If ioWPosOffset is negative or ioWPosOffset plus ioReqCount is beyond the end of the drive, your driver must fail the request with a paramErr.

Supporting Large Volumes in Your Driver

To support large volumes correctly, your driver must implement the following:

• Your driver must return true in response to the kdgWide Driver Gestalt selector. You may want to use the GetDriverGestaltBooleanResponse macro to ensure that you set the correct response byte in the parameter block.
• When handling all _Read or _Write requests, your driver must check whether the kUseWidePositioning flag is set in ioPosMode. If it is, you must cast the parameter block to an XIOParam and do the I/O at the 64-bit offset specified in ioWPosOffset. This type of request is known as a wide request.
• If kUseWidePositioning is not set, your driver must do the I/O at the offset specified by dCtlPosition. You must cast this signed value to an unsigned quantity (UInt32) to correctly handle offsets from 2 GB to 4 GB. This type of request is known as a narrow request.

There are some important caveats of which you should be aware.

• There is no guarantee that the system will check with Driver Gestalt before issuing a wide request. The system expects that any driver controlling a drive larger than 4 GB will respond to wide request correctly. Similarly, the system expects that a driver controlling a drive whose size is between 2 GB and 4 GB is smart enough to treat dCtlPosition as unsigned.
• There is no guarantee that the system will always use wide requests when talking to a drive larger than 4 GB. In fact, the system currently decides on a request-by-request basis whether to use a wide or a narrow request, based on the request's offset on the drive. However, you must not rely on this behavior; you must handle wide requests to offsets less than 4 GB correctly.
• The dCtlPosition field of the DCE is a 32-bit quantity, thus it cannot accurately reflect the position of the current I/O beyond the 4 GB boundary. You should ignore dCtlPosition for wide requests and use it only for narrow requests.

Notes for Developers Calling Disk Drivers

If you're writing software that issues _Read or _Write requests to a disk driver, you must be careful to avoid some common pitfalls. Specifically, you should follow the recommendations given below.

• You should always use an ioPosMode of fsFromStart when calling a disk driver. Because dCtlPosition cannot accurately reflect the position beyond 4 GB, other positioning modes do not work as expected in all cases.
• Before issuing a wide request, you should call Driver Gestalt to determine whether the driver supports wide requests.
• If the driver supports wide requests, you may choose to always use wide requests for that driver. However, for maximum compatibility, DTS recommends that you take the same approach as the system by deciding to use a wide or narrow request based on the offset into the drive.

The following code snippet implements these recommendations.

static void SetWidePosOffset(UInt32 blockOffset, XIOParamPtr pb)
    // Set up ioPosMode and either ioPosOffset or ioWPosOffset for a
    // device _Read or _Write.
{
    pb->ioWPosOffset.lo = blockOffset << 9;  // convert block number
    pb->ioWPosOffset.hi = blockOffset >> 23; // to wide byte offset

    if ( pb->ioWPosOffset.hi != 0 ) {
  // Offset on drive is >= 4G, so use wide positioning mode
  pb->ioPosMode = fsFromStart | (1 << kWidePosOffsetBit);
    } else {
  // Offset on drive is < 4G, so use regular positioning mode,
  // and move the offset into ioPosOffset
  pb->ioPosMode = fsFromStart;
  ((IOParam *)pb)->ioPosOffset = pb->ioWPosOffset.lo;
    }
}

In addition, you should never call PBReadImmed or PBWriteImmed on a disk driver unless you know, in advance, that the disk driver supports such requests. Many disk drivers fail to handle Immediate requests properly. Because immediate requests result in the disk driver possibly being reentered, these problems are hard to detect and debug.

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How the ROM Loads SCSI and ATA Drivers

This section describes how the ROM loads SCSI and ATA drivers from a driver partition. Understanding this process is critical to an understanding of the chaining driver architecture, and useful for general disk driver writers.

When a Macintosh boots, code in the ROM scans each SCSI and ATA bus for block devices in a bus-specific manner. Once it has found a potentially bootable block device, the ROM attempts to load a driver from that device. The ROM executes the following procedure to load a driver.

1. It first reads device block 0 of the disk. This is the driver descriptor map (DDM) and is structured as the Block0 data type defined in "SCSI.h". It checks that block 0 is a valid DDM by comparing the sbSig field to sbSIGWord ($4552 or 'ER'). If the DDM is not valid, the ROM ignores the device.
2. It then reads device block 1 of the disk and looking for the first entry of the partition map. A partition map entry is represented by the Partition data structure in "SCSI.h". For the partition to be recognized, the pmSig field must be newPMSigWord ($5453 or 'PM'). The ROM uses the pmMapBlkCnt field of this first partition to determine the size of the partition map as a whole.
3. The ROM then searches the DDM for the first driver that is compatible with this bootable bus. The DDM contains an array of DDMap structures. The key field in this structure is ddType, which identifies the type of driver defined by the structure. If the device is attached to a SCSI bus, the ROM looks for a DDMap whose ddType is kDriverTypeMacSCSI. If the device is attached to an ATA bus, the ROM looks for a DDMap whose ddType is kDriverTypeMacATA.
4. The ROM then searches (by reading consecutive device blocks) the partition map for the chosen driver's partition map entry (whose pmParType starts with "Apple_Driver" and whose pmPyPartStart equals the ddBlock field of the DDMap of the chosen driver). It stores this partition map entry in a temporary memory block.
5. The ROM then searches (by reading consecutive device blocks) the partition map for the first HFS partition (whose pmParType is "Apple_HFS"). It stores this partition map entry in a temporary memory block.
6. The ROM then uses the driver's DDMap to read the driver into memory. It first allocates a pointer block in the system heap to hold the driver (the size of this block is the size of the driver in blocks (ddSize) multiplied by the disk's block size (sbBlkSize)) and then reads the driver off the disk (starting from ddBlock) into that buffer.
7. Next, the ROM checksums the driver to ensure its validity. For more information on the exact details of the checksum, see Driver Checksums.
8. The ROM then calls the driver's entry point. The exact calling conventions are described below. The driver is expected to install itself in the unit table, open itself, and create drive queue elements for each mountable partition on the disk. (The exact definition of "mountable" is covered in Cooperating with File System Manager.)

If any of these steps fail, the ROM assumes that the device is not bootable and attempts to boot from the next available device.

If you want to support the Macintosh Plus in your driver, you need to be aware of the subtle difference between it and later computers. Specifically the buffer pointed to by A0 when the Macintosh Plus ROM calls your driver contains the contents of the second block on the disk (the old style "device partition map"); on all subsequent computers, the buffer pointed to by A0 contains the first "Apple_HFS" partition map entry.

Each driver has two possible entry points. The primary entry point is at the beginning of the memory block holding the driver. The secondary entry point is 8 bytes into the memory block holding the driver. In general, the primary entry point is called when an "old" driver is loaded, or a "new" driver is loaded by an 'old' ROM, and the secondary entry point is used when a 'new' ROM load a "new" driver. The secondary entry point has extra parameters that make sense in the 'new' ROM environment.

The exact definition of "old" and "new" depends on the bootable bus. For SCSI, a "new" ROM is one that contains SCSI Manager 4.3, and a "new" driver is indicated by the bytes "43" in the two bytes following the "Apple_Driver" in pmParType. For ATA, an 'old' ROM is one that contains ATA Manager 1.0. All newer versions of ATA Manager use the secondary entry point. A 'new' ATA Manager will always call the secondary entry point of the driver.

Both entry points use register-based calling conventions. The register usage is shown in the table below:

Register D3

Old Apple SCSI drivers require that register D3 be set to a non-zero value in order to boot correctly. This bug was fixed in September 1996 although, if you are writing a SCSI disk -mounting utility, you may still encounter these old drivers.

Register D5

The data in register D5 depends on both the bootable bus and the entry point called. The following table indicates the possible combinations.

The format of DeviceIdentATA is given below.

struct DeviceIdentATA {
    UInt8 diReserved;
    UInt8 busNum;
    UInt8 devNum;
    UInt8 diReserved2;
};
typedef struct DeviceIdentATA   DeviceIdentATA;
typedef DeviceIdentATA *  DeviceIdentATAPtr;

The fields have the following meaning:

diReserved

Reserved. When calling a disk driver, the ROM sets this to 0; however, in the case described below, this field contains meaningful data.

busNum

The ATA bus number.

devNum

If the machine has ATA 0/1 support, this is the device number of the device on that bus. Otherwise, it must be zero.

diReserved2

Reserved. Set to 0.

In some cases (such as the entry point to a patch loaded by the Apple patch driver), the diReserved field is used to distinguish between a DeviceIdent and DeviceIdentATA. The appropriate values for this field are given below.

enum {
    kBusTypeSCSI  = 0,
    kBusTypeATA= 1,
    kBusTypePCMCIA= 2,
    kBusTypeMediaBay    = 3
};

enum {
    kSecondaryEntryPointCalled  = 29,    // 1 => secondary entry point called
    kDontMountVolumes  = 30,    // 1 => don't mount any partitions
    kAfterSystemStartupTime     = 31     // 1 => post-system startup load
};

However, in the circumstances described above, all bits in register D5 can be used to hold information. Therefore, DTS recommends that you discontinue the practice of storing flags in the high bit of D5 where practical.

A good substitute for the kAfterSystemStartupTime flag is described in Disk Drivers and the System Heap.

Register D0

The significance of register D0 on return from your driver's entry point varies depending on the manager that loaded your driver.

• For ATA Manager, your driver should return an error result in the low word of register D0 and, if the driver successfully installed, its driver reference number in the high word (or a high word of zero otherwise). If you return an error value other than noErr, ATA Manager will unload your driver code from memory.
• For SCSI Manager 4.3, the contents of register D0 are always ignored. SCSI Manager 4.3 will never unload your driver from memory. With some clever coding, you can unload the bulk of your driver code upon a failed installation, if you feel that level of polish is necessary.
• For old SCSI Manager, the situation varies depending on the particular ROM.
  • The Mac Plus will treat register D0 as an error result and unload your driver if you return a non-zero value.
  • Subsequent computers ignore the contents of register D0. If your driver fails to install and you want its code to be unloaded, you can return to the return address plus 4 bytes, which signals this to SCSI Manager. Doing this on a computer running SCSI Manager 4.3 will crash the system.

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Loading FireWire Drivers

This section is only available under non-disclosure agreement. Please contact DTS for details.

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Chaining Drivers and Patch Partitions

Booting a computer is always a tricky exercise. One of the perennial challenges is working around problems in the ROM that prevent the OS from booting far enough to load patches in the normal way. On pre-ROM-in-RAM Macintosh computers, this problem is solved by means of chaining drivers and patch partitions. Patches loaded in this way have been used to:

• support booting from volumes larger than 2 GB on machines that don't have such support in the ROM (for example, NuBus-based Power Macintoshes),
• fix bugs in the ROM SCSI Manager that would otherwise prevent booting, and
• provide support for snag booting, where the user can hold down the C key to force the system to boot from the CD-ROM device.

This section explains how chaining drivers and patch partitions are implemented, and how you can license chaining drivers and patches suitable for inclusion in your own disk formatting utility.

Background Material

This section presumes that you are familiar with the existing documentation on disk partitions and how Mac OS loads a driver from the disk at startup time. Specifically, you should be familiar with:

• Inside Macintosh: Devices, Chapter 3 "SCSI Manager" The Structure of Block Devices (page 3-12 through 3-15) and Data Structures, (page 3-23 through 3-27), and
• Inside Macintosh: Devices, Chapter 4 "SCSI Manager 4.3" Loading and Initializing a Driver (page 4-11)
• Secrets of the Partition Map, earlier in this document.

Architecture Overview

When it boots from a block device, Mac OS loads the driver from the device itself. This driver is held in a driver partition (whose pmParType starts with "Apple_Driver") and is referenced by an entry in the driver descriptor map (DDM), which is stored in the first device block on the disk. The ROM searches the DDM to find the appropriate driver, loads that driver into memory, and calls it.

The chaining driver architecture works by installing a special driver in place of the standard disk driver. This chaining driver performs its operation (typically it applies a patch to the ROM) and then loads the next suitable driver in the DDM, in exactly the same way as the ROM would have. The next driver may be a real disk driver, or yet another chaining driver.

The sequence of drivers loaded in this way is known as a driver chain. There may be more than one driver chain on the disk; often, there is one for each bootable bus possible for that disk. For example, a Zip disk may have a chain of SCSI drivers (whose pmParType is "Apple_Driver43") for use when the Zip disk is inserted in a SCSI Zip device, and a chain of ATA drivers (whose pmParType is "Apple_Driver_ATA") for use when the Zip disk is inserted in an ATA Zip device.

The last driver in a driver chain does not need to support chaining because there is nothing to chain to. This means that you don't need to modify your disk driver to support this architecture, as long as the disk driver is always installed last in the chain.

One special kind of chaining driver is the patch driver. This is a driver supplied by Apple that is responsible for loading and executing system patches out of a patch partition. Each patch has a patch descriptor, which contains a four character code that uniquely identifies the patch. Once it has loaded the patches, the patch driver chains to the next driver, as any other chaining driver would.

In general, you do not need to write a patch driver, or the patches it installs. However, your formatting utility must install the patch driver and the patch partition such that the right patches are loaded.

Available Patches

Apple supplies both patch drivers and patches to developers. The available patch drivers are listed below:

• "PatchChainDriver" -- This patch driver is used when booting from a SCSI device.
• "ATAPatchChainDriver" -- This patch driver is used when booting from an ATA device, such as an internal ATA hard disk.
• "ATAPIPatchChainDriver" -- This patch driver is used when booting from an ATAPI device, such as an ATAPI CD-ROM.

The following patches are available.

• 'mesh' -- This patch fixes a bug in the ROM SCSI Manager's handling of the MESH chip. It is required to successfully boot on a machine with that chip.
• 'scsi' -- This patch makes adjustments to the classic SCSI Manager to enable booting from CD-ROM devices.
• 'ruby' -- This patch installs support for volumes larger than 2 GB on machines that don't have this support in the ROM.
• 'snag' -- This patch implements the "To start up from this CD-ROM, hold down the C key as the computer starts up" functionality used in many bootable CD-ROM products. It is only necessary on pre-ROM-in-RAM computers; ROM-in-RAM computers implement snag booting in Open Firmware.

To legally include these patch drivers and patches in your formatting software, you must license the patches from Apple. Contact Apple Software Licensing for details.

Advice for Formatting Utilities

The first thing that a formatting utility must do is decide how many driver chains need to be constructed. This is determined by the number of possible bootable buses for the disk. For example, a SCSI device can only be attached via SCSI, so the utility need only construct one driver chain. In contrast, an removable cartridge disk might be placed in either a SCSI or ATA mechanism, and therefore must contain two driver chains, one for SCSI and one for ATA. Moreover, a PowerBook internal ATA hard disk device needs to have a SCSI driver chain if it is to work in target mode.

For each driver chain constructed, the formatting utility must first create a partition for the patch driver and then create a partition for the disk driver itself. When creating partitions, the formatting utility must be careful to write the driver signature into the pmPad field of the Partition record. Chaining drivers (including the patch driver) need this signature to correctly find the next driver to load. The utility should also be sure to set up the pmPartStatus field according to the description in pmPartStatus Revealed.

In addition to creating the driver partitions, the formatting utility must also create entries in the DDM with the appropriate driver type. See New Driver Types for a list of driver types, and Architecture in Detail for an explanation of the relationship between them.

The formatting utility must also construct the "Apple_Patches" partition. Some rules must be observed when doing this.

• The pmPartName field of the partition map entry should be "Patch Partition".
• The pmParType field of the partition map entry must be "Apple_Patches".
• The first block of the patch partition contains a list of the patches in the partition.
• Patches are run in order, so it is necessary to place patches that are critical to the correct operation of later patches (like 'mesh' and 'scsi') before the less critical ones (like 'snag').
• Patch descriptors contain a version number. The formatting utility should not replace a newer patch with an older one.
• Patch descriptors are variable length data structures. You cannot index the list of patches as an array.

There are also some non-obvious factors when deciding whether to install a particular patch on a particular disk.

• The MESH patch ('mesh') should be installed on any disk which might be booted from via SCSI. In particular, the MESH patch is required on the internal ATA hard disk on PowerBooks, because it is possible they might be used to boot a machine while in target mode.
• The large volume support patch ('ruby') is only required if any of the partitions on the disk are 2 GB or larger.
• Do not install the 'snag' patch on hard disks! Doing so will prevent the user from snag booting a CD. This is because, if the C key is held down, the hard disk 'snag' patch prevents booting from the CD, while the CD-ROM 'snag' patch prevents booting from the hard disk.

Finally, formatting utilities should aim to leave some free space in the partition maps, driver partitions, and patch partitions. Drivers and patches grow over time and wasting a few KB now may radically ease the job of upgrading a driver or patch in the future.

Architecture in Detail

This section describes the chaining driver architecture in detail, including how chaining drivers intercept the driver loading process, the Apple patch driver, and the structure of the patches it loads. To understand this section, you need to understand how the ROM loads SCSI and ATA drivers.

Pre-Chaining Example

The following diagram shows how a partition map might be laid out prior to the introduction of chaining drivers. This example includes both ATA and SCSI drivers, a setup which is useful for disks that can be mounted in both ATA and SCSI mechanisms. Some salient features are:

• The sample SCSI driver has a driver signature of 'QSCZ', and the sample ATA driver has a driver signature of 'QATA'.

Chaining Drivers

The basic idea behind chaining drivers is very simple. A chaining driver appears to the ROM as the actual disk driver. It has a DDM entry of the appropriate type (kDriverTypeMacSCSI for SCSI, kDriverTypeMacATA for ATA) and it has a partition with the appropriate type ("Apple_Driver43" for SCSI, "Apple_Driver_ATA" for ATA). The ROM finds, loads, and executes the chaining driver as if it was the real disk driver. The chaining driver does its operation (patching, password protection, and so on) and then finds, loads and executes the next driver in the driver chain. This process is repeated once for each driver in the chain.

The first chaining driver in a driver chain always has the ddType expected by the ROM (kDriverTypeMacSCSI for SCSI, kDriverTypeMacATA for ATA). Subsequent drivers in the driver chain have their ddType set to the two's complement of the standard value (kDriverTypeMacSCSIChained for SCSI, kDriverTypeMacATAChained for ATA).

There are a number of important implementation details for chaining drivers.

• All drivers in the chain, except the first, must have the kPartitionIsChainCompatible bit set in the pmPartStatus field of their partition map entries to indicate that they can be chained to (they don't have to be loaded directly by the ROM). The first driver may have this bit set, although it is not required.
• A chaining driver must always have the kPartitionCanChainToNext bit set in the pmPartStatus field of its partition map entry. While this bit is not actually needed for the chaining driver to be loaded, formatting utilities may use the bit to determine the required order of drivers in the DDM.
• A chaining driver may also contain the real disk driver. If it does, it should have the kPartitionIsRealDeviceDriver bit set in the pmPartStatus field of its partition map entry.
• The ROM loads the chaining driver exactly as it would a normal driver. Therefore, if a chaining SCSI driver wants to have its checksum validated by the ROM, it must set the first four bytes of its partition map entry pmPartName field to "Maci".
• A chaining driver must find the next driver to load using the following algorithm.
  1. First, the chaining driver should search the partition map for its own partition map entry. It can distinguish itself from other drivers by looking for its driver signature in the pmPad field.
  2. Then, the driver should look up its entry in the DDM. It can find itself by matching the pmPyPartStart field of its partition map entry to the ddBlock field of its DDMap.
  3. It can then find the DDMap of the next driver in the driver chain by searching onwards from its own DDMap for a DDMap with the appropriate ddType. In this case, appropriate is either the two's complement of the chaining driver's ddType (if the chaining driver is first in the chain), or the same ddType as the chaining driver (if the chaining driver is subsequent in the chain).
• There may be no next driver to load. The chaining driver should treat this as an error, and handle it as described below.
• A chaining driver must load and execute the next driver exactly as the ROM would have. The exact details are covered in the previous section. Note that the chaining driver must:
  1. checksum the driver, as described in Driver Checksums, and
  2. remember which of its entry point was called (primary or secondary) and call the same entry point for the next driver, and
  3. call the next driver with registers A0, D5, and D7 set exactly as they were when the chaining driver was called.
  4. handle any error returned by the next driver as described below.
• A chaining driver may need to increase the size of the system heap to allow it to allocate enough memory to load the next driver. See Disk Drivers and the System Heap for details on doing this.

How the chaining driver handles errors depends on whether the chaining driver precedes the disk driver in the driver chain. If the chaining driver precedes the disk driver, any error loading the next driver, or any error returned by the next driver's entry point, is fatal. The chaining driver should return ioErr from its entry point. However, if the chaining driver is the disk driver (both kPartitionCanChainToNext and kPartitionIsRealDeviceDriver are set in its pmPartStatus) or comes after the disk driver, any error loading the next driver is not fatal, and the chaining driver should return noErr regardless of any error loading the next driver in the chain.

The following diagram shows how a partition map might be laid out for a disk that can only be booted on an ATA bus and which has a chaining driver. Some salient features are:

• The DDM has the chaining driver first, followed by the disk driver (with a negated ddType).
• The chaining flags are set in the pmPartStatus fields of the chaining driver's and the disk driver's partition map entry.

The Apple Patch Driver

The Apple patch driver is a chaining driver supplied by Apple that loads patches from a special partition on the disk. You must license the patch driver and its accompanying patches for inclusion with your disk driver software. This section describes the operation of the patch driver insofar as is necessary for you to write a formatting utility that correctly installs the patches.

Typically, the patch driver is installed first in the driver chain. It finds the patch partition by searching the partition map for an entry whose type is "Apple_Patches". It then walks the patch partition, loading and executing the patches. Finally, it chains to the next driver.

The patch partition is structured to contain multiple patches. The first block of the patch partition contains a patch list, a description of all the patches in the partition. The patch list is defined by the PatchList structure.

struct PatchList {
    UInt16 numPatchBlocks;
    UInt16 numPatches;
    PatchDescriptor thePatch[1];
};
typedef struct PatchList    PatchList;
typedef PatchList *PatchListPtr;

The fields have the following meaning:

numPatchBlock

The number of device blocks used to hold the patch list. The patch driver must load this many blocks from the start of the patch partition to ensure that it has all the patch descriptors.

numPatches

The number of patch descriptors contained in the patch list.

thePatch

The patch descriptor describing the first patch in the patch list.

Each patch in the patch list is described by the PatchDescriptor data type.

struct PatchDescriptor {
    OSType patchSig;
    UInt16 majorVers;
    UInt16 minorVers;
    UInt32 flags;
    UInt32 patchOffset;
    UInt32 patchSize;
    UInt32 patchCRC;
    UInt32 patchDescriptorLen;
    Str32  patchName;
    UInt8  patchVendor[1];
};
typedef struct PatchDescriptor  PatchDescriptor;
typedef PatchDescriptor * PatchDescriptorPtr;
typedef PatchDescriptorPtr *    PatchDescriptorHandle;
 
enum {
    kRequiredPatch = 0x00000001;
};

The fields have the following meaning:

patchSig

A four-character code that uniquely identifies the patch. If you create your own patches, you must use a registered creator code.

majorVers

A major version number. Typically this is 1.

minorVers

A minor version number. Typically this is 0. This combines with the major version number to indicate a version of the form 1.0, 1.1, and so on.

flags

A set of flags for the patch. The only bit currently defined is kRequiredPatch. If this is set, the patch must succeed for the system to continue booting. See the section on error handling below. All other bits are reserved and must be set to zero.

patchOffset

The offset, in device blocks, from the beginning of the patch partition to the patch code.

patchSize

The actual size of the patch code in bytes.

patchCRC

A checksum for the patch. This is calculated using the 16-bit driver checksum algorithm.

patchDescriptorLen

The total length, in bytes, of this patch descriptor. The minimum value for this field is sizeof(PatchDescriptor), which is 62 bytes. This value of this field must be even.

patchName

A human-readable name for the patch. This name is never displayed to users or used by the system. It is present for debugging and diagnosis only.

patchVendor

A human-readable description of the patch vendor. This name is never displayed to users or used by the system. It is present for debugging and diagnosis only. This string may be followed by an arbitrary amount of patch-specific data.

In addition, because of the aforementioned error, the minimum value for the patchDescriptorLen field is 62, not 61 as previously documented.

When the patch driver executes a patch, it does so by creating a new pointer block in the system heap which is large enough to hold the patch, reading the patch code into that block, and then calling the patch entry point (the first byte of the memory block) using the calling conventions described in the next section.

As part of its operation, the patch driver increases the size of the system heap to accommodate the size of the patches loaded.

Patch Driver Error Handling

Error handling in the patch driver follows the general outline for error handling in chaining drivers. Specifically, an error is classified as either fatal or non-fatal. For a fatal error, the patch driver discards the current patch descriptor and patch code (if any) and returns ioErr from its entry point, which indicates to the system that this disk is unusable. Fatal errors include:

• failure to load a required patch (one whose patch descriptor's flags field has kRequiredPatch set),
• a positive result from a required patch,
• a negative error result from any patch, and
• failure to load the next driver (the patch driver is always loaded first in the driver chain, so a failure to load the next driver is always a fatal error).

For a non-fatal error, the patch driver simply discards the patch descriptor and patch code for the patch and continues trying to load the next patch (if any) or the next driver in the driver chain. Non-fatal errors include:

• inability to load a non-required patch, and
• a positive error result from a non-required patch.

Patch Execution

The prototype for a patch's entry point is given below.

extern pascal OSErr MyPatch(PatchDescriptorPtr myPatch,
 DeviceIdent myDevID);

The parameters to the entry point are:

myPatch

A pointer to the patch's patch descriptor. The patch can use this pointer to extract patch-specific information from patchVendor part of the patch descriptor. The memory containing the patch descriptor will be deallocated after the patch returns; the patch is responsible for copying any information it needs to retain.

myDevID

A device identifier which identifies the device from which the patch was loaded. The diReserved field of this parameter can be used to distinguish whether this is a SCSI DeviceIdent or a DeviceIdentATA.

result

noErr, if the patch was successful. The patch driver will dispose of the patch descriptor but leave the patch code in memory. A positive error code, if the patch encountered a non-fatal error. A negative error code, if the patch encountered a fatal error. See the description of patch driver error handling for details.

The patch's code is always loaded in the system heap. The patch's entry point is always called at system task time.

• A patch should not assume that it was loaded from an ATA or SCSI device. For example, a SCSI-specific patch should behave correctly if it is loaded from an ATA device. This can happen if the patch is installed on a removable cartridge disk that can be mounted in both ATA or SCSI devices.
• A patch should not assume the existence of optional system software capabilities. For example, a SCSI Manager 4.3 specific patch should not assume that SCSI Manager 4.3 is present. It is possible for an external device to be moved from a machine with SCSI Manager 4.3 to a machine without it, and vice versa.
• Because of the above, patches should avoid loading data from the disk. If your patch needs data, you should add the data after the patchVendor field of your patch descriptor.
• Patches are loaded very early in the startup sequence and must allocate memory as outlined in Disk Drivers and the System Heap.
• A patch should work correctly even if it is loaded twice. For example, if the same patch is installed on multiple SCSI devices, both patches will be executed at startup time and the patches must coordinate to avoid any conflicts.

Because a patch's code is always loaded in a pointer block in the system heap, it can reduce its size in memory using clever code sorting and SetPtrSize. For example, imagine a patch that has 5 KB of install code and 25 KB of resident code. The patch can reduce its memory footprint by sorting the code as shown below.

The following code snippet shows how this might be achieved in C.

extern pascal OSErr MyPatch(PatchDescriptorPtr myPatch,
 DeviceIdent myDevID)
{
    OSErr err;

    err = MyInstall(myPatch, myDevID);
    SetPtrSize( (Ptr) &MyPatch,
 (UInt32) &MyInstall - (UInt32) &MyPatch
     );
    return err;
}

Putting It All Together

The following diagram shows the layout of a disk that can be booted via SCSI and ATA.

• The DDM has two patch chains, one for ATA booting and one for SCSI booting.
• Each patch chain starts with the appropriate Apple patch driver.
• The first block of the "Apple_Patches" partition contains a list of patches to be installed on the machine. The remaining blocks contain the code for the patches themselves.
• The 'mesh' patch is installed to ensure correct operation when booted via SCSI on a machine with the MESH chip.
• The 'ruby' patch is installed to allow booting on machines without large volume support in ROM. Note that the total disk size in this example is 5 GB. On the smaller disks used in the previous examples, the 'ruby' patch would not be necessary.

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Disk Drivers and the System Heap

Disk drivers typically allocate their memory in the system heap. A disk driver must use one of three techniques to allocate system heap space, depending on the execution context. There are three relevant execution contexts for your driver:

1. Driver Load Time -- If you driver is bootable, it is called at driver load time to install itself in the unit table.
2. System Startup -- It is possible for your driver to be called at system startup time, after driver load time but before system startup is complete. For example, if your driver sets dNeedTime and some startup code (for example, an 'INIT') brings up a dialog, your driver will receive accRun requests.
3. After System Startup -- System startup time finishes when the Process Manager starts and launches the Finder.

The best way to detect whether system startup is complete is to compare the first byte (the length) of the Pascal string returned by LMGetCurApName to $FF. If the first byte is $FF, the system is still starting up. If it is any other value, system startup is complete.

There is no good way to distinguish between driver load time and system startup time. Your driver must remember internally whether it is executing as a result of its install routine being called.

Driver Load Time

At driver load time, a driver that needs to allocate a large amount of memory must grow the system heap using SetApplBase. This system routine is documented as Inside Macintosh: Memory, along with a warning that applications should not use it. However, it is expected that disk drivers which need to expand the system heap will use this routine.

A simple example of calling SetApplBase is shown below.

static void ExpandSystemHeap(Size bytesToGrow)
{
    THz currentZone;
 
    // Only try to expand the system heap if we're at startup time,
    // ie the CurApName is still filled with $FFs.

    assert( LMGetCurApName()[0] == 0xFF );    // from <assert.h>
 
    currentZone = GetZone();
 
    // Round up the request to 512 bytes.

    bytesToGrow = (bytesToGrow + 0x01FF) & ~0x01FF;

    // Set the system heap to the specified size.

    SetApplBase((Ptr)((UInt32) (LMGetSysZone())->bkLim + bytesToGrow));

    SetZone(currentZone);
}

System Startup

Disk drivers that load as part of the 'INIT' loading process should request that the system heap be grown using a 'sysz' resource, as documented in Inside Macintosh: Memory and Inside Macintosh: Operating System Utilities, and amended in Technote IM 2 Inside Macintosh: Memory Errata.

After Startup Time

After the system has started up, a disk driver should allocate its system heap memory using NewPtrSys, or NewHandleSys. The system heap will automatically expand to meet these requirements.

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PowerPC Native Disk Drivers

Many developers wish to implement their disk drivers in PowerPC native code. However, there is no well-defined architecture for native disk drivers. There are a number of consequences and drawbacks, which this section discusses in detail.

The Need for Speed

Most drivers are I/O bound. They spend a small amount of time setting up an I/O request and a proportionally much larger amount of time waiting for the underlying hardware to complete that request. Such drivers receive very little benefit from executing as native code. Moreover, the benefit varies depending on the ratio of small I/O requests (which tend to be CPU bound) to large I/O requests (which tend to be I/O bound).

On the other hand, some drivers are CPU bound. For example, a driver that encrypts data as it transfers it to the disk may spend a significant amount of time executing driver code. This may even be true for a complex, but still I/O focused driver, such as a RAID driver or a caching disk driver. These drivers may receive significant benefit from "going native."

The only good way to tell whether your driver receives a benefit from conversion to native code, and that the benefit is enough to overcome the difficulties in doing so, is to actually profile the code. You may be able to do this quickly by profiling the driver code in an application framework before facing the challenges of creating a working native driver.

Difficulties with Taking Your Driver Native

The primary difficulty in creating a native disk driver is that there is no well-defined architecture for it. The PCI-native driver model has a number of drawbacks for disk driver developers.

1. It does not include a disk driver I/O family expert. It is possible to write a generic native driver (kServiceCategoryNdrvDriver) which acts as a disk driver, but it is not possible to do so within the native driver architecture. Specifically, a disk driver must link to InterfaceLib to access routines like AddDrive. Linking to InterfaceLib works just fine on the current Mac OS, but it is not legal within the native driver model and guarantees that your driver will not be compatible with any future Mac OS that emulates this model on a non-traditional framework.
2. The PCI native driver model is not available on older, non-PCI-based, Power Macintosh computers.

Another possible approach is to implement a partially native driver, where code that you know to take a long time is implemented as native code. This makes a lot of sense in some cases, such as an encryption driver, where the lengthy code is easily isolated from the rest of the driver.

It is also possible to implement a virtually fully native driver without the PCI native driver module, using only a tiny amount of 68K glue code to provide the driver header and an interface to IODone. In general, this approach is not recommended by DTS because of the complexities involved in transitioning from 68K to native code and back.

When taking a disk driver native, it is important to remember that the primary client of the disk driver is the File Manager, which is not native. While it is likely that a disk driver will incur Mixed Mode switches regardless of whether it is native or not (the SCSI Manager and ATA Manager are native), taking the driver native shifts the line where the switches occur, and may increase or decrease the number of switches depending on how your driver works. So, to guarantee an overall speed improvement, it is important that the native driver be significantly faster than the emulated one.

Native Drivers and accRun

Before implementing a disk driver as a native driver, you must read DTS Q&A DV 35, "Native Drivers ('ndrv's) and dNeedTime", which describes an incompatibility between native drivers and dNeedTime.

The rest of this technote assumes that you are building a 68K driver, and thus you can set dNeedTime in dCtlFlags to get system task time via the accRun Control request. If you are building a native driver and you need system task time, you must implement one of the alternative mechanisms described in the Q&A.

68K drivers should continue to use dNeedTime as always.

Recommendations

DTS does not recommend that developers implement disk drivers in PowerPC native code unless there is clear evidence that doing so improves the performance significantly. Typically this is only for drivers that are CPU bound, such as encrypting drivers. A standard SCSI or ATA driver is I/O bound, and receives little benefit from running native.

The easiest way to implement a PowerPC native driver is using the native driver model, introduced with the PCI-based Power Macintosh computers. However, this approach will not work on older Power Macintosh computers. Another recommended alternative is to implement a partially native driver, where core functionality (such as an encryption engine) is in native code.

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Installing and Removing Drivers and Drives

Over the course of the past 15 years, Mac OS has evolved from a relatively static environment -- a Mac with one or two floppy drives that needed to be connected at startup time -- to a highly dynamic system, where devices and disks come and go at runtime. The Mac OS disk driver architecture has, to a large extent, coped with this evolution, as long as driver writers play by the rules. This section explains these rules in detail.

Installing and Removing Drivers

There are a number of ways to install your disk driver.

1. If you're writing a native driver that controls a real piece of hardware (a FireWire device, or a PCI RAM disk card, for example), you can set up your DriverDescription so that the system automatically finds and opens your device driver. See Designing PCI Cards and Drivers for Power Macintosh Computers for details.
2. If you're writing a native driver with no corresponding hardware, you can use DriverLoaderLib to install your driver directly. See Designing PCI Cards and Drivers for Power Macintosh Computers for details.
3. If you're writing a 68K driver, you should use TradDriverLoaderLib to install your driver. Installing a driver in the unit table is easy to do half right but tricky to do exactly right, which is why DTS strongly recommends that developers use TradDriverLoaderLib. The only exception is boot disk drivers, where the limited scope of the task makes the general nature of TradDriverLoaderLib seem a little too much. See Code Sharing for more details on this.

To remove a disk driver from the unit table, you have a number of choices.

1. If the driver is a native driver, you must use the DriverLoaderLib routine RemoveDriver to remove it.
2. If the driver is a 68K driver, you should have installed it using TradDriverLoaderLib. If so, you can remove it using the TradRemoveDriver routine provided by that library.
3. If the driver was not installed using TradDriverLoaderLib (either because it was a boot disk driver or because it wasn't installed by your software), you should follow the procedure described in the Hostile Takeovers section of this document.

Code Sharing

Code sharing is a technique used by some third-party disk drivers to share the device driver code between multiple drivers in the unit table. Code sharing is a legal technique, although it is not implemented by Apple disk drivers and is not recommended by DTS. Before shipping a driver that uses code sharing, you need to understand the costs and benefits of the technique.

How Code Sharing Works

The basic algorithm for code sharing is as follows:

1. When your driver installs itself, it first scans the unit table to see whether another instance of it is already installed.
2. If there is an existing instance, you must check its version number to determine whether to use its code or replace its code with the code in your driver. You can get the driver's version using a Driver Gestalt kdgVersion request.
3. If the existing driver is older, you must somehow dispose of its code and replace it with yours. As there is no Apple-defined way of replacing 'DRVR's, you must use a private hand-off technique built in to your driver. Alternatively, you might consider not sharing code in this case.
4. If the existing driver is newer, you must somehow inform it that another instance of it is being created. Again, there is no Apple-defined technique for this; this information exchange is private to your driver.

In addition, drivers that implement code sharing must reference count the code in order to support close and purge correctly.

The Pros and Cons of Code Sharing

Code sharing has one big advantage: it reduces memory usage if two devices controlled by your driver are attached to the system. This may be especially significant for a complex device driver, such as a RAID driver.

The disadvantages of code sharing include:

• The standard library for installing 'DRVR's, TradDriverLoaderLib does not support code sharing. If you implement code sharing, you must do this leg work yourself.
• Supporting code sharing significantly complicates the installation code path of your driver. As the installation code is run very early in the startup sequence, bugs in that code are often very hard to debug.
• Drivers that use code sharing cannot be reopened.

Managing Drive Queue Elements

The Basics

The drive queue and its associated drive queue elements are documented in Inside Macintosh: Files, page 2-85. However, that document does not describe how drive queue elements are created, installed, removed, and destroyed.

Your disk driver must add a drive queue element for each file system partition on each disk it controls. The strategy you use for managing drive queue elements is largely up to you, within some basic constraints. Drive queue elements must be allocated in the system heap, primarily so that they persist throughout the life of the system but also, in the case of paging devices, so that they are held resident in memory. Typically, your driver is responsible for creating and disposing the drive queue elements under your control.

One popular technique for managing drive queue elements is to extend the DrvQEl data structure with the extra per-drive storage needed by your driver. This makes it easy for you to find your per-drive storage structure given either the DrvQElPtr (just cast the DrvQElPtr to a pointer to your per-drive storage structure) or the drive number (search the drive queue looking for that drive number, which gives you the DrvQElPtr, and then proceed as before).

Another important thing to remember about drive queue elements is that the system requires that you implement four flag bytes immediately before the first field of the DrvQEl. You can choose to either define these flags as part of your per-drive storage structure (which complicates the cast between it and a DrvQElPtr), or just handle those flags as a special case.

When creating a drive queue element, you must first decide on the drive number for the new drive. The algorithm to find a free drive number is very simple: start with drive number 5 (or, by convention, 8 if you're a hard disk driver), check to see whether it is in use, and if so, increment the number and try again.

Once your driver has created a drive queue element, it can put it in the drive queue with the system routine AddDrive. AddDrive is a very thin wrapper around GetDrvQHdr and Enqueue. It is not strictly necessary to use this routine, but it may be convenient.

Once your driver has created a drive queue element, it should inform the system of its existence, as described in Cooperating with File System Manager.

Removing a Drive Queue Element

Removing a drive queue element is somewhat more convoluted than adding one. The basics are very simple. The system doesn't define a RemoveDrive routine; you must remove a drive queue element using the code shown below. Compilable source is available as part of the MoreDisks module of the DTS sample code library MoreIsBetter.

extern pascal OSErr MoreRemoveDrive(DrvQElPtr drvQEl)
{
    OSStatus err;

    if ( MoreVolumeMountedOnDrive(drvQEl->dQDrive, false) == 0 ) {
  err = Dequeue( (QElemPtr) drvQEl, GetDrvQHdr());
    } else {
  err = volOnLinErr;
    }
    return err;
}

It is also important to remember that, if your disk driver can be called asynchronously, it is possible for even synchronous requests to be executed at interrupt time. See Technote 1067, "Traditional Device Drivers: Sync or Swim."

Consequently, your driver should never add or remove a drive queue element except in its Open or Close entry point, or in response to an immediate request that it knows was made at system task time, such as an accRun Control request. In particular, it is not safe for your disk driver to remove a drive queue element as part of handling an kEject Control request.

If your disk driver needs to remove a drive queue element, it must mark the drive queue element as "to be removed" and set dNeedTime in its dCtlFlags. When it receives the accRun Control request, it must walk the drive queue looking for drives it owns that are marked as "to be removed" and remove them there. The DTS sample AsyncDriverSample shows a correct implementation of this.

Drive Queue Strategies

While removing a drive queue element is relatively simple, deciding on a strategy for when to remove the drive queue element is not. The key is how you handle the kEject Control request. The two common strategies are described below.

Real Block Device

If your disk driver controls some real piece of hardware (for example, a floppy drive, a SCSI ejectable disk device, a SCSI fixed disk device), you should not remove the drive queue element when the user ejects the disk. You should leave the drive queue element in the queue so that, when the user reinserts the disk, you can post a "disk inserted" event for it. This simplifies your life and ensures that your drive's drive number is relatively stable.

This approach may seem a little strange for fixed disks, but it works just fine. Fixed disks are typically not marked as ejectable, so the user can not really eject a fixed disk; they simply unmount the volume mounted on it. This is useful for programs (for example, a disk recovery program) which want to unmount a volume, perform some low-level activity on the disk, and then remount the volume. To remount the volume, the program can simply call PBMountVol for the old drive number. This technique would not be possible if the fixed disk driver removed its drive queue elements when the disk was ejected.

So leaving fixed disk drive queue elements in the drive queue is not only safe, it is also convenient.

Virtual Block Device

If you are writing a disk driver for some virtual block device (like a RAM disk, or a disk image, or a block-oriented network protocol), your job is more complex. In the simple case, if the disk is ejected when there is no volume mounted on it, you should remove the drive queue element, as explained in the previous section.

However, if the disk is ejected while there is still a volume mounted on it, you must take special action to avoid the disk switch dialog asking the user to insert the virtual disk. [The "Please insert disk RAM Disk" disk switch dialog is particularly amusing or annoying depending on how much caffeine you've had that day.] There are two common ways to prevent this:

1. Non-Ejectable -- You can mark your virtual drive as non-ejectable. This is probably the easiest and most sensible approach. It can; however, have problems when running with virtual memory enabled on older systems. Old versions of the Virtual Memory Manager assume that any local, non-ejectable drive is eligible for paging. This may not be true for your virtual block device driver (especially if it relies on the network). Modern versions of the Virtual Memory Manager (starting with Mac OS 8.1) query the drive, via Driver Gestalt (kdgVMOptions), to see whether the drive is really suitable for paging. However, for older systems, the only recourse you have is to make your drive as ejectable.
2. Auto Reinsert -- If you are forced to mark your virtual drive as ejectable, the following algorithm will ensure that you remove the drive queue element when appropriate and never have an ejected drive with a volume mounted on it:
   1. When you receive the kEject Control request, mark the drive as not having a disk in place and set the dNeedTime bit in the dCtlFlags.
   2. When the system sends you an accRun, walk the drive queue looking for any of your drives which are marked as not having a disk in place.
   3. For those drives, walk the system VCB queue looking for a volume that has been ejected but was previously mounted on that drive.
   4. If you find such a volume, post a "disk inserted" event for that drive. This will remount the volume back on the drive.
   5. If you don't find such a volume, remove the drive queue element for that drive.

   The DTS sample AsyncDriverSample implements this algorithm.

Hot Swapping

The Mac OS I/O subsystem is evolving towards more support for hot-swappable devices. Modern I/O buses, like USB and FireWire, fully support the addition and removal of devices while the system is running.

Unfortunately, other parts of Mac OS are not as friendly to the hot swapping of devices. For disk devices, hot swapping is a relatively new idea, and Mac OS support for hot swappable disk devices is limited. While it is possible to add new drives on the fly, removing a drive while there is a volume mounted on it will cause the system to crash, with possible loss of user data.

There are two basic strategies for handling a disk device being unplugged unexpectedly.

1. Put It Back -- If possible, your disk driver should stop the system and post a dialog telling the user to replace the disk device. This dialog should have no OK or Cancel buttons; the user must replace the device to continue using the system and the dialog should auto-dismiss when the device is reattached. This is tricky to implement, for the following reasons.
   • In most cases, the notification that a device has been removed happens at interrupt time, and it is unsafe to pose a standard Dialog Manager dialog at interrupt time. You can defer the dialog until your next accRun, but you may receive I/O requests before you are issued an accRun, and you must be prepared to handle those I/O requests at interrupt time.
   • Some I/O families are not capable of handling reconnections at interrupt time.
   • Some block devices are not tagged with a unique ID so, even if the device is reconnected, there is no way to guarantee that it is the same device.
2. Error Everything -- Your device driver should simply fail all I/O requests with the error driverHardwareGoneErr (-503). In Mac OS 9.0 and higher, the File Manager recognizes this error and responds in the following way.
   • It sets the kVCBFlagsHardwareGoneBit in the vcbFlags field of the Volume Control Block (VCB).
   • It posts a Notification Manager alert saying, "The device for disk 'MyDiskName' was unexpectedly disconnected. To prevent data loss, always use the Finder to 'Put Away' a disk before disconnecting its disk device."
   • At system task time, it walks the volume list looking for volumes that have the kVCBFlagsHardwareGoneBit bit set and puts them offline.
3. This approach is similar to that taken by the AppleShare external file system when the connection to the server tears.

In some cases, your I/O family may provide support for the hot unplugging of disk devices. For example, if your device is connected via the media bay, the system will automatically put up a "put it back" dialog for you, and if your device is connected via FireWire, you can use the FWWaitForDeviceRePlug routine to wait for a device to be reconnected.

You might think to use the same technique as the media bay but this is unsatisfactory for a number of reasons:

• It is not supported by DTS.
• The System Error Handler uses QuickDraw to display its dialogs. Calling QuickDraw at interrupt time is illegal, and therefore calling SysError at interrupt time is illegal. This is a known compromise in the design of SysError and is acceptable because, when you're handling a real system error, the system is already in a precarious state. However, using SysError as part of the standard operation of your disk driver is asking for trouble.

In the absence of an I/O family-specific solution, the best compromise solution is to implement the following algorithm:

• When you are notified of a device being disconnected, check whether there is a volume mounted on any of its drives. If there isn't a volume mounted on any of its drives, all is well; you can simply wait for the next accRun to remove the device's drive queue elements. If there is a volume mounted, set a flag in your per-drive storage.
• If you receive an I/O request while that flag is set, fail the request with driverHardwareGoneErr error. On Mac OS 9.0 or above, this is a sufficient response. On earlier systems, you should also:
  • At accRun time, look through for drives owned by your driver which have the flag set. For each missing device, post a Dialog Manager dialog that requires the user to reattach the device. Once the device is reattached, clear the flag and return from your accRun handler.
  • Post a Notification Manager alert like that described above.

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Close and Purge

For maximum friendliness, your driver must support being closed. This section explains how to support the Close request properly in you disk driver and how a formatting utility might use this to allow a disk to be reformatted without rebooting.

Supporting Close in Your Driver

Your driver must support the Close request properly. This requirement was documented a long time ago and is as true today as it ever was.

Your driver's Close entry point should attempt to undo all the things that its Open entry point did, including the tasks listed below.

1. Check to see whether there are volumes mounted in any of the drives controlled by the driver. Code for doing this is shown below. If there are, the Close should fail with a closErr.

extern pascal SInt16 MoreVolumeMountedOnDrive(SInt16 drive,
Boolean ejectedIsMounted)
{
    SInt16 result;
    VCBPtr thisVCB;
 
    result = 0;
    thisVCB = (VCBPtr) GetVCBQHdr()->qHead;
    while (thisVCB != nil && result == 0) {
  if (thisVCB->vcbDrvNum == drive ||
 (ejectedIsMounted &&
  thisVCB->vcbDrvNum == 0 &&
  thisVCB->vcbDRefNum == drive
 )
  ) {
   result = thisVCB->vcbVRefNum;
  } else {
   thisVCB = (VCBPtr) thisVCB->qLink;
  }
    }

    return result;
}

1. Terminate all asynchronous operations and remove any interrupt handlers. Your Close entry point is always called immediately at system task time, so it is safe to "spin wait" (that is, synchronously wait) for asynchronous operations to complete.
2. Remove all of its drive queue elements from the drive queue. The system supplies a routine for adding a drive queue elements (AddDrive), but not one to remove them. The code for removing a drive queue element is shown earlier.
3. Unregister with any system services with which it registered. Typically, this includes SCSI Manager or ATA Manager, Power Manager, and Shutdown Manager.
4. Free any memory allocated by the driver, including the dCtlStorage.

If it is absolutely impossible to complete any of these steps, the driver should return closErr and continue as if the close had not been requested.

In addition, your dri