source: trunk/minix/lib/i386/string/memcmp.s@ 9

!       memcmp()                                        Author: Kees J. Bot
!                                                               2 Jan 1994
.sect .text; .sect .rom; .sect .data; .sect .bss

! int memcmp(const void *s1, const void *s2, size_t n)
!       Compare two chunks of memory.
!
.sect .text
.define _memcmp
        .align  16
_memcmp:
        cld
        push    ebp
        mov     ebp, esp
        push    esi
        push    edi
        mov     esi, 8(ebp)     ! String s1
        mov     edi, 12(ebp)    ! String s2
        mov     ecx, 16(ebp)    ! Length
        cmp     ecx, 16
        jb      cbyte           ! Don't bother being smart with short arrays
        mov     eax, esi
        or      eax, edi
        testb   al, 1
        jnz     cbyte           ! Bit 0 set, use byte compare
        testb   al, 2
        jnz     cword           ! Bit 1 set, use word compare
clword: shrd    eax, ecx, 2     ! Save low two bits of ecx in eax
        shr     ecx, 2
        repe
        cmps                    ! Compare longwords
        sub     esi, 4
        sub     edi, 4
        inc     ecx             ! Recompare the last longword
        shld    ecx, eax, 2     ! And any excess bytes
        jmp     last
cword:  shrd    eax, ecx, 1     ! Save low bit of ecx in eax
        shr     ecx, 1
        repe
    o16 cmps                    ! Compare words
        sub     esi, 2
        sub     edi, 2
        inc     ecx             ! Recompare the last word
        shld    ecx, eax, 1     ! And one more byte?
cbyte:  test    ecx, ecx        ! Set 'Z' flag if ecx = 0
last:   repe
        cmpsb                   ! Look for the first differing byte
        seta    al              ! al = (s1 > s2)
        setb    ah              ! ah = (s1 < s2)
        subb    al, ah
        movsxb  eax, al         ! eax = (s1 > s2) - (s1 < s2), i.e. -1, 0, 1
        mov     edx, esi        ! For bcmp() to play with
        pop     edi
        pop     esi
        pop     ebp
        ret