1 /*
2  *	mpvecsub(mpdigit *a, int alen, mpdigit *b, int blen, mpdigit *diff)
3  *
4  *		diff[0:alen-1] = a[0:alen-1] - b[0:blen-1]
5  *
6  *	prereq: alen >= blen, diff has room for alen digits
7  */
8 .text
9 
10 .p2align 2,0x90
11 .globl mpvecsub
12 	.type mpvecsub, @function
13 mpvecsub:
14 	/* Prelude */
15 	pushl %ebp
16 	movl %ebx, -4(%esp)		/* save on stack */
17 	movl %esi, -8(%esp)
18 	movl %edi, -12(%esp)
19 
20 	movl	8(%esp), %esi		/* a */
21 	movl	16(%esp), %ebx		/* b */
22 	movl	12(%esp), %edx		/* alen */
23 	movl	20(%esp), %ecx		/* blen */
24 	movl	24(%esp), %edi		/* diff */
25 	subl	%ecx,%edx
26 	xorl	%ebp,%ebp			/* this also sets carry to 0 */
27 
28 	/* skip subraction if b is zero */
29 	testl	%ecx,%ecx
30 	jz	_sub1
31 
32 	/* diff[0:blen-1],borrow = a[0:blen-1] - b[0:blen-1] */
33 _subloop1:
34 	movl	(%esi, %ebp, 4), %eax
35 	sbbl	(%ebx, %ebp, 4), %eax
36 	movl	%eax, (%edi, %ebp, 4)
37 	incl	%ebp
38 	loop	_subloop1
39 
40 _sub1:
41 	incl	%edx
42 	movl	%edx,%ecx
43 	loop	_subloop2
44 	jmp done
45 
46 	/* diff[blen:alen-1] = a[blen:alen-1] - 0 */
47 _subloop2:
48 	movl	(%esi, %ebp, 4), %eax
49 	sbbl	$0, %eax
50 	movl	%eax, (%edi, %ebp, 4)
51 	INCL	%ebp
52 	LOOP	_subloop2
53 
54 done:
55 	/* Postlude */
56 	movl -4(%esp), %ebx		/* restore from stack */
57 	movl -8(%esp), %esi
58 	movl -12(%esp), %edi
59 	movl %esp, %ebp
60 	leave
61 	ret
62