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Inside Macintosh: Mac OS Runtime Architectures /
Chapter 2 - Indirect Addressing in the CFM-Based Architecture

PowerPC Implementation

The PowerPC implementation of the indirect addressing model is fairly straightforward. The PowerPC runtime environment uses general-purpose register GPR2 as the base register. Note that the use of this register to access the direct data area is a convention, not a requirement. Debuggers and other analytical applications should not assume that GPR2 is the base register.

Historically (from IBM documentation) the set of pointers to a fragment's indirectly accessed data was referred to as the Table of Contents and its base register was called the Table of Contents Register (RTOC).
To access imported data or indirect global data, the build-time offset of the global data item is added to the value in GPR2. The result is the address of a pointer that points to the desired data.

To access imported routines, the offset of the routine is added to the value in GPR2, as in the data version, but the result points not directly to the routine, but to a transition vector.

The PowerPC transition vector typically contains two elements. The first must be the address of the routine being called. By convention the second element contains the address of the called fragment's direct data area.

Prior to preparation, the transition vector contains

During preparation, the Code Fragment Manager adds the code and data section start addresses to the offset values, generating absolute addresses for the routine and the location of the direct data area. Figure 2-4 shows the unprepared and prepared versions of the transition vector.

Figure 2-4 Unprepared and prepared PowerPC transition vectors

The transition vector may contain any number of 4-byte fields. Currently only the first two are used. During an indirect call, GPR12 is assumed to point to the transition vector itself; this convention allows the called routine to access any additional fields in the transition vector beyond the first two.
In order for indirect calls to work properly, certain requirements must be met on the part of the calling routine and the called routine. These requirements are as follows:

How these requirements are implemented is determined by convention. For example, in the PowerPC runtime environment, glue code in the calling routine handles loading the proper values into GPR2 and GPR12. Any other actions are also determined by convention.

Glue Code for Named Indirect Calls
Glue Code for Pointer-Based Calls

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11 MARCH 1997

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