minix/lib/libc/arch/i386/misc/oneC_sum.S

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/* oneC_sum() - One complement`s checksum Author: Kees J. Bot */
/* 9 May 1995 */
/* See RFC 1071, "Computing the Internet checksum" */
/* See also the C version of this code. */
.text
.globl _oneC_sum
.balign 16
_oneC_sum:
push %ebp
movl %esp, %ebp
push %esi
push %edi
movzwl 8(%ebp), %eax /* Checksum of previous block */
movl 12(%ebp), %esi /* Data to compute checksum over */
movl 16(%ebp), %edi /* Number of bytes */
xorl %edx, %edx
xorb %cl, %cl
align:
testl $3, %esi /* Is the data aligned? */
je aligned
testl %edi, %edi
je 0f
movb (%esi), %dl /* Rotate the first unaligned bytes */
decl %edi /* into the edx register */
0:
incl %esi
rorl $8, %edx
rorl $8, %eax /* Rotate the checksum likewise */
addb $8, %cl /* Number of bits rotated */
jmp align
aligned:
addl %edx, %eax /* Summate the unaligned bytes */
adcl $0, %eax /* Add carry back in for one`s complement */
jmp add6test
.balign 16
add6:
addl (%esi), %eax /* Six times unrolled loop, see below */
adcl 4(%esi), %eax
adcl 8(%esi), %eax
adcl 12(%esi), %eax
adcl 16(%esi), %eax
adcl 20(%esi), %eax
adcl $0, %eax
addl $24, %esi
add6test:
subl $24, %edi
jae add6
addl $24, %edi
jmp add1test
.balign 16
add1:
addl (%esi), %eax /* while ((edi -= 4) >= 0) */
adcl $0, %eax /* eax += *esi++; */
addl $4, %esi /* edi += 4; */
add1test:
subl $4, %edi
jae add1
addl $4, %edi
je done /* Are there extra bytes? */
movl (%esi), %edx /* Load extra bytes in a full dword */
andl mask-4(,%edi,4), %edx /* Mask off excess */
addl %edx, %eax /* Add in the last bits */
adcl $0, %eax
done:
roll %cl, %eax /* Undo the rotation at the beginning */
movl %eax, %edx
shrl $16, %eax
addw %dx, %ax /* Add the two words in eax to form */
adcw $0, %ax /* a 16 bit sum */
pop %edi
pop %esi
pop %ebp
ret
#ifdef __ACK__
.rom
#else
.data
#endif
.balign 4
mask:
.long 0x000000FF, 0x0000FFFF, 0x00FFFFFF
/* */
/* $PchId: oneC_sum.ack.s,v 1.2 1996/03/12 19:33:51 philip Exp $ */