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/* mpvecsub(mpdigit *a, int alen, mpdigit *b, int blen, mpdigit *diff) */
/* diff[0:alen-1] = a[0:alen-1] - b[0:blen-1] */
/* prereq: alen >= blen, diff has room for alen digits */
/* (very old gnu assembler doesn't allow multiline comments) */
.text
.p2align 2,0x90
.globl _mpvecsub
_mpvecsub:
/* Prelude */
pushl %ebp /* save on stack */
pushl %ebx
pushl %esi
pushl %edi
leal 20(%esp), %ebp /* %ebp = FP for now */
movl 0(%ebp), %esi /* a */
movl 8(%ebp), %ebx /* b */
movl 4(%ebp), %edx /* alen */
movl 12(%ebp), %ecx /* blen */
movl 16(%ebp), %edi /* diff */
subl %ecx,%edx
xorl %ebp,%ebp /* this also sets carry to 0 */
/* skip subraction if b is zero */
testl %ecx,%ecx
jz 2f
/* diff[0:blen-1],borrow = a[0:blen-1] - b[0:blen-1] */
1:
movl (%esi, %ebp, 4), %eax
sbbl (%ebx, %ebp, 4), %eax
movl %eax, (%edi, %ebp, 4)
incl %ebp
loop 1b
2:
incl %edx
movl %edx,%ecx
loop 3f
jmp 4f
/* diff[blen:alen-1] = a[blen:alen-1] - 0 */
3:
movl (%esi, %ebp, 4), %eax
sbbl $0, %eax
movl %eax, (%edi, %ebp, 4)
incl %ebp
loop 3b
4:
/* Postlude */
popl %edi
popl %esi
popl %ebx
popl %ebp
ret
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