source: trunk/minix/lib/i386/string/memset.s@ 10

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

! void *memset(void *s, int c, size_t n)
!       Set a chunk of memory to the same byte value.
!
.sect .text
.define _memset
        .align  16
_memset:
        push    ebp
        mov     ebp, esp
        push    edi
        mov     edi, 8(ebp)     ! The string
        movzxb  eax, 12(ebp)    ! The fill byte
        mov     ecx, 16(ebp)    ! Length
        cld
        cmp     ecx, 16
        jb      sbyte           ! Don't bother being smart with short arrays
        test    edi, 1
        jnz     sbyte           ! Bit 0 set, use byte store
        test    edi, 2
        jnz     sword           ! Bit 1 set, use word store
slword: movb    ah, al
        mov     edx, eax
        sal     edx, 16
        or      eax, edx        ! One byte to four bytes
        shrd    edx, ecx, 2     ! Save low two bits of ecx in edx
        shr     ecx, 2
        rep
        stos                    ! Store longwords.
        shld    ecx, edx, 2     ! Restore low two bits
sword:  movb    ah, al          ! One byte to two bytes
        shr     ecx, 1
        rep
    o16 stos                    ! Store words
        adc     ecx, ecx        ! One more byte?
sbyte:  rep
        stosb                   ! Store bytes
done:   mov     eax, 8(ebp)     ! Return some value you have no need for
        pop     edi
        pop     ebp
        ret