Register Conventions

The compiler has different register conventions, depending on the target processor used; some of the registers have specific uses during code generation. The following section describes the generic names of the registers on a platform per platform basis. It also indicates what registers are used as scratch registers, and which can be freely used in assembler blocks.

accumulator register

The accumulator register is at least a 32-bit integer hardware register, and is used to return results of function calls which return integral values.

accumulator 64-bit register

The accumulator 64-bit register is used in 32-bit environments and is defined as the group of registers which will be used when returning 64-bit integral results in function calls. This is a register pair.

float result register

This register is used for returning floating point values from functions.

self register

The self register contains a pointer to the actual object or class. This register gives access to the data of the object or class, and the VMT pointer of that object or class.

frame pointer register

The frame pointer register is used to access parameters in subroutines, as well as to access local variables. References to the pushed parameters and local variables are constructed using the frame pointer. ³.

stack pointer register

The stack pointer is used to give the address of the stack area, where the local variables and parameters to subroutines are stored.

scratch registers

Scratch registers are those which can be used in assembler blocks, or in external object files without requiring any saving before usage.

Processor mapping of registers

This indicates what registers are used for what purposes on each of the processors supported by Free Pascal. It also indicates which registers can be used as scratch registers.

Intel 80x86 version

Intel 80x86 Register table

Generic register name	CPU Register name
accumulator	EAX
accumulator (64-bit) high / low	EDX:EAX
float result	FP(0)
self	ESI
frame pointer	EBP
stack pointer	ESP
scratch regs.	N/A
OS_128 type (128 bit integer)	RDX:RAX

Motorola 680x0 version

Motorola 680x0 Register table

Generic register name	CPU Register name
accumulator	D0
accumulator (64-bit) high / low	D0:D1
float result	FP04
self	A5
frame pointer	A6
stack pointer	A7
scratch regs.	D0, D1, A0, A1, FP0, FP1