Intel x86-64 XSAVE/XRSTOR

Question

I\'m a CS student writing in Intel x86-64 assembly, compiling with nasm, and running on an Core i7 processor with Ubuntu 12.04 as the guest OS. Does anyone hav

Answer 1

The xsave/xrstor/xsaveopt instructions are used to perform a full save/restore of the extended state in the processor to/from memory. Similar to fxsave/fxrstor, it saves/restores fpu state st[0..7], xmm[0..7], mxcsr, etc... in addition to supporting ymm[0..15] and future extensions (zmm[0..31]). The actual values saved, and the data layout are enumerated via the relevant cpuid leaves. The use is generally operating system context switching. The programmer reference describes how to use them correctly.

For general userspace register save/restore, the assembler usually has a facility for saving/restoring a set of registers.

For example...

masm

foo PROC USES eax,ebx,ecx
    xor    ebx, ebx

loop:
    mov    eax, [esi + ebx*4]
    mov    [edi + ebx*4], eax
    inc    ebx
    dec    ecx

    jnz    loop

    ret

foo ENDP

yasm

%macro  mpush 1-*

  %rep  %0
        push    %1
  %rotate 1
  %endrep

%endmacro

%macro  mpop 1-*

  %rep %0
  %rotate -1
        pop     %1
  %endrep

%endmacro

foo:
    mpush  rax,rbx,rcx
    xor    rbx, rbx

loop:
    mov    rax, [rsi + rbx*8]
    mov    [rdi + rbx*8], rax
    inc    rbx
    dec    rcx

    jnz    loop

    mpop   rax,rbx,rcx
    ret

In ia-32, there is a pushad to save all the general purpose registers, but with amd64 you need to have corresponding push/pop pairs for each of the registers you use.