Register usage

x86-32 provides a grand total of eight integer registers, a full 7 of which are usable as integer registers!! Whatever will we do with such bounty. Table 3 summarizes these registers.

Table 3: Possible x86-32 registers
%eax integer return val NO
%edx dividend reg NO
%ecx count reg NO
%ebp optional frame pointer YES
%ebx local reg YES
%esi local reg YES
%edi local reg YES
%esp Stack pointer YES
%st fp return val, aliased with mmx NO
%st1-7 scratch regs, aliased with mmx NO
%mmx0-7 scratch MMX regs, aliased with fp stack NO
%xmm0-7 scratch SSE/SSE2 regs NO

Note that the MMX registers are only available on architectures possing the MMX ISA extensions, and that the xmm registers are only available on architectures implementing SSE or SSE2. It is also worth mentioning that all of the integer registers with the exception of the stack pointer (%esp) may be used as general purpose registers, in addition to their specialized usages. Finally, please note that the MMX registers and floating point are aliased, so they cannot be referenced at the same time.

Registers marked as CALLEE SAVE = YES must be explicitly saved by any functions which write them. Registers marked NO are scratch registers whose values need not be saved. EAX is used by intergral functions to return the value, and the floating point top-of-stack, %st returns floating point values. EBP is documented as base pointer, but I think most sane people just do all referencing from ESP (the stack pointer), and utilize EBP as an additional integer register.

As for floating point registers, the x86-32 has eight of them arranged in a pseudo-stack. A document this brief cannot explain this travesty to you if you don't already grok it. Go read the x87 FPU description in an assembler book, and come back when you are through crying.

Clint Whaley 2012-07-10