Specifications

A Minimal PowerPCª Boot Sequence for

Executing Compiled C Programs

PowerPC EABI Compliance

Part III PowerPC EABI Compliance

The PowerPC EABI speciÞes the system interface for compiled programs. The EABI is based on the

System

V Application Binary Interface

and the

PowerPC Processor Supplement

. For general ABI documentation,

refer to these documents, as well as the

PowerPC Embedded Application Binary Interface

. This document

only includes aspects of the EABI that apply directly to preparing to jump into a compiled C program.

For running compiled programs, the EABI-speciÞed register conventions must be followed. The EABI

deÞnes how the processorÕs registers are to be used by a conforming application. Table 5 lists the register

conventions for the PowerPC EABI.

The symbols _SDA_BASE and _SDA2_BASE are deÞned during linking. They specify the locations of the

small data areas. A program must load these values into GPR13 and GPR2, respectively, before accessing

program data.

The small data areas contain part of the data of the executable. They hold a number of variables that can be

accessed within a 16-bit signed offset of _SDA_BASE or _SDA2_BASE. References to these variables are

performed through references to GPR13 and GPR2 by the user program. Typically, the small data areas

contain program variables that are less than or equal to 8 bytes in size, although this differs by compiler. The

variables in SDA2 are read-only.

Before executing user code, the startup code must also set up the stack pointer in GPR1. This pointer must

be 8-byte aligned for the EABI (as opposed to 16-byte aligned for the PowerPC ABI) and should point to

the lowest allocated valid stack frame. The stack grows toward lower addresses, so its location should be

selected so that it does not grow into data or bss areas.

Table 5. PowerPC EABI Registers

GPR1 Stack Frame Pointer

GPR2 _SDA2_BASE

GPR13 _SDA_BASE

GPR31 Local variables or environment pointer

GPR0 VolatileÑmay be modiÞed during linkage

GPR3, GPR4 VolatileÑused for parameter passing and return values

GPR5ÐGPR10 Used for parameter passing

GPR11ÐGPR12 VolatileÑmay be modiÞed during linkage

GPR14ÐGPR30 Used for local variables

FPR0 Volatile register

FPR1 VolatileÑused for parameter passing and return values

FPR2ÐFPR8 VolatileÑused for parameter passing

FPR9ÐFPR13 Volatile registers

FPR14ÐFPR31 Used for local variables