# memcmp() Author: Kees J. Bot
# 2 Jan 1994
# int memcmp(const void *s1, const void *s2, size_t n)
# Compare two chunks of memory.
#
.intel_syntax
.globl _memcmp
.text
.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
cmpsd # 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
cmpsw # 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