What registers must be preserved by an x86 function?

Question

I\'m writing a function in x86 assembly that should be callable from c code, and I\'m wondering which registers i have to restore before i return to the caller.

Curr

Answer 1

if you are unsure about the registers' situation, these instructions below could save the day easily.

PUSHA/PUSHAD -- Push all General Registers
POPA/POPAD -- Pop all General Registers

These instructions push and pop the general purpose and SI/ESI , DI/EDI registers in certain order.

The order for PUSHA/PUSHAD instruction is as follows.

Opcode  Instruction  Clocks   Description

60      PUSHA        18       Push AX, CX, DX, BX, original SP, BP, SI, and DI
60      PUSHAD       18       Push EAX, ECX, EDX, EBX, original ESP, EBP ESI, and EDI

And the order for POPA/POPAD instruction is as follows. (in reverse order)

Opcode   Instruction   Clocks   Description

61       POPA          24       Pop DI, SI, BP, SP, BX, DX, CX, and AX
61       POPAD         24       Pop EDI, ESI, EBP, ESP(***),EBX, EDX, ECX, and EAX

*** The ESP value is discarded instead of loaded into ESP.