minix/lib/i86/misc/oneC_sum.s

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2005-04-21 16:53:53 +02:00
! oneC_sum() - One complement`s checksum Author: Kees J. Bot
! 23 May 1998
! See RFC 1071, "Computing the Internet checksum"
! See also the C version of this code.
.text
.define _oneC_sum
.align 4
_oneC_sum:
push bp
mov bp, sp
push si
push di
mov ax, 4(bp) ! Checksum of previous block
mov si, 6(bp) ! Data to compute checksum over
mov di, 8(bp) ! Number of bytes
xor dx, dx
xorb cl, cl
align: test si, #1 ! Is the data aligned?
jz aligned
test di, di
jz 0f
movb dh, (si) ! First unaligned byte in high half of
dec di ! the dx register, i.e. rotate 8 bits
0: inc si
movb cl, #8 ! Number of bits "rotated"
ror ax, cl ! Rotate the checksum likewise
aligned:add ax, dx ! Summate the unaligned byte
adc ax, #0 ! Add carry back in for one`s complement
jmp add6test
.align 4
add6: add ax, (si) ! Six times unrolled loop, see below
adc ax, 2(si)
adc ax, 4(si)
adc ax, 6(si)
adc ax, 8(si)
adc ax, 10(si)
adc ax, #0
add si, #12
add6test:
sub di, #12
jae add6
add di, #12
jmp add1test
.align 4
add1: add ax, (si) ! while ((di -= 2) >= 0)
adc ax, #0 ! ax += *si++;
add si, #2 ! di += 2;
add1test:
sub di, #2
jae add1
add di, #2
jz done ! Is there an extra byte?
movb dl, (si) ! Load extra byte in word
xorb dh, dh
add ax, dx ! Add in the last bits
adc ax, #0
done:
rol ax, cl ! Undo the rotation at the beginning
pop di
pop si
pop bp
ret