page	,132
	title	memcmp - compare to blocks of memory
;***
;memcmp.asm - compare two blocks of memory
;
;	Copyright (c) 1985-2001, Microsoft Corporation. All rights reserved.
;
;Purpose:
;	defines memcmp() - compare two memory blocks lexically and
;	find their order.
;
;Revision History:
;	05-16-83  RN	initial version
;	07-20-87  SKS	rewritten for speed
;	05-17-88  SJM	Add model-independent (large model) ifdef
;	08-04-88  SJM	convert to cruntime/ add 32-bit support
;	08-23-88  JCR	386 cleanup
;	10-25-88  JCR	General cleanup for 386-only code
;	03-23-90  GJF	Changed to _stdcall. Also, fixed the copyright.
;	05-10-91  GJF	Back to _cdecl, sigh...
;	05-01-95  GJF	New, faster version from Intel!
;	05-09-95  GJF	Replaced a bad ret with a good jz retnull (my editing
;			error, not Intel's).
;       11-06-95  GJF   Corrected order in which esi and edi are saved.
;
;*******************************************************************************

	.xlist
	include cruntime.inc
	.list

page
;***
;int memcmp(buf1, buf2, count) - compare memory for lexical order
;
;Purpose:
;	Compares count bytes of memory starting at buf1 and buf2
;	and find if equal or which one is first in lexical order.
;
;	Algorithm:
;	int
;	memcmp (buf1, buf2, count)
;		char *buf1, *buf2;
;		unsigned count;
;	{
;		if (!count)
;			return(0);
;		while (--count && *buf1 == *buf2)
;			{
;			buf1++;
;			buf2++;
;			}
;		return(*buf1 - *buf2);
;	}
;
;Entry:
;	char *buf1, *buf2 - pointers to memory sections to compare
;	unsigned count - length of sections to compare
;
;Exit:
;	returns -1 if buf1 < buf2
;	returns  0 if buf1 == buf2
;	returns +1 if buf1 > buf2
;
;Uses:
;
;Exceptions:
;
;*******************************************************************************

	CODESEG

	public	memcmp
memcmp	proc

	.FPO	( 0, 3, 0, 0, 0, 0 )

	mov	eax,[esp+0ch]	; eax = counter
	test	eax,eax 	; test if counter is zero
	jz	short retnull	; return 0

	mov	edx,[esp+4]	; edx = buf1
	push	esi
	push	edi
	mov	esi,edx		; esi = buf1
	mov	edi,[esp+10h]	; edi = buf2

; Check for dword (32 bit) alignment
	or	edx,edi
	and	edx,3		; edx=0 iff buf1 are buf2 are aligned
	jz	short dwords
 
; Strings are not aligned. If the caller knows the strings (buf1 and buf2) are
; different, the function may be called with length like -1. The difference 
; may be found in the last dword of aligned string, and because the other 
; string is misaligned it may cause page fault. So, to be safe. the comparison
; must be done byte by byte.
	test	eax,1
	jz	short main_loop
 
	mov	cl,[esi]
	cmp	cl,[edi]
	jne	short not_equal
	inc	esi
	inc	edi
	dec	eax
	jz	short done	; eax is already 0

main_loop:
	mov	cl,[esi]
	mov	dl,[edi]
	cmp	cl,dl
	jne	short not_equal
 
	mov	cl,[esi+1]
	mov	dl,[edi+1]
	cmp	cl,dl
	jne	short not_equal
 
	add	edi,2
	add	esi,2
 
	sub	eax,2
	jnz	short main_loop
done:
	pop	edi
	pop	esi
retnull:
	ret			; _cdecl return
 
 
dwords:
	mov	ecx,eax
	and	eax,3		; eax= counter for tail loop
 
	shr	ecx,2
	jz	short tail_loop_start
				; counter was >=4 so may check one dword
	rep	cmpsd
 
	jz	short tail_loop_start
 
; in last dword was difference
	mov	ecx,[esi-4]	; load last dword from buf1 to ecx
	mov	edx,[edi-4]	; load last dword from buf2 to edx
	cmp	cl,dl		; test first bytes
	jne	short difference_in_tail
	cmp	ch,dh		; test seconds bytes
	jne	short difference_in_tail
	shr	ecx,10h
	shr	edx,10h
	cmp	cl,dl		; test third bytes
	jne	short difference_in_tail
	cmp	ch,dh		; they are different, but each one is bigger?
;	jmp	short difference_in_tail
 
difference_in_tail:
	mov	eax,0
				; buf1 < buf2 buf1 > buf2
not_equal:
	sbb	eax,eax		; AX=-1, CY=1 AX=0, CY=0
	pop	edi		; counter
	sbb	eax,-1		; AX=-1	AX=1
	pop	esi
	ret			; _cdecl return
 
; in tail loop we test last three bytes (esi and edi are aligned on dword
; boundary)
tail_loop_start:
 
	test	eax,eax 	; eax is counter%4 (number of bytes for tail
				; loop)
	jz	short done	; taken if there is no tail bytes
	mov	edx,[esi]	; load dword from buf1
	mov	ecx,[edi]	; load dword from buf2
	cmp	dl,cl		; test first bytes
	jne	short difference_in_tail
	dec	eax		; counter--
	jz	short tail_done
	cmp	dh,ch		; test second bytes
	jne	short difference_in_tail
	dec	eax		; counter--
	jz	short tail_done
	and	ecx,00ff0000h	; test third bytes
	and	edx,00ff0000h
	cmp	edx,ecx
	jne	short difference_in_tail
	dec	eax
tail_done:
	pop	edi
	pop	esi
	ret			; _cdecl return
 
memcmp	endp
	end