| 1 | ! oneC_sum() - One complement`s checksum Author: Kees J. Bot
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| 2 | ! 23 May 1998
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| 3 | ! See RFC 1071, "Computing the Internet checksum"
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| 4 | ! See also the C version of this code.
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| 5 |
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| 6 | .text
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| 7 |
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| 8 | .define _oneC_sum
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| 9 | .align 4
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| 10 | _oneC_sum:
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| 11 | push bp
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| 12 | mov bp, sp
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| 13 | push si
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| 14 | push di
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| 15 | mov ax, 4(bp) ! Checksum of previous block
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| 16 | mov si, 6(bp) ! Data to compute checksum over
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| 17 | mov di, 8(bp) ! Number of bytes
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| 18 |
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| 19 | xor dx, dx
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| 20 | xorb cl, cl
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| 21 | align: test si, #1 ! Is the data aligned?
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| 22 | jz aligned
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| 23 | test di, di
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| 24 | jz 0f
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| 25 | movb dh, (si) ! First unaligned byte in high half of
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| 26 | dec di ! the dx register, i.e. rotate 8 bits
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| 27 | 0: inc si
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| 28 | movb cl, #8 ! Number of bits "rotated"
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| 29 | ror ax, cl ! Rotate the checksum likewise
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| 30 | aligned:add ax, dx ! Summate the unaligned byte
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| 31 | adc ax, #0 ! Add carry back in for one`s complement
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| 32 |
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| 33 | jmp add6test
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| 34 | .align 4
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| 35 | add6: add ax, (si) ! Six times unrolled loop, see below
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| 36 | adc ax, 2(si)
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| 37 | adc ax, 4(si)
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| 38 | adc ax, 6(si)
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| 39 | adc ax, 8(si)
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| 40 | adc ax, 10(si)
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| 41 | adc ax, #0
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| 42 | add si, #12
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| 43 | add6test:
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| 44 | sub di, #12
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| 45 | jae add6
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| 46 | add di, #12
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| 47 |
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| 48 | jmp add1test
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| 49 | .align 4
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| 50 | add1: add ax, (si) ! while ((di -= 2) >= 0)
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| 51 | adc ax, #0 ! ax += *si++;
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| 52 | add si, #2 ! di += 2;
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| 53 | add1test:
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| 54 | sub di, #2
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| 55 | jae add1
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| 56 | add di, #2
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| 57 |
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| 58 | jz done ! Is there an extra byte?
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| 59 | movb dl, (si) ! Load extra byte in word
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| 60 | xorb dh, dh
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| 61 | add ax, dx ! Add in the last bits
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| 62 | adc ax, #0
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| 63 | done:
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| 64 | rol ax, cl ! Undo the rotation at the beginning
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| 65 | pop di
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| 66 | pop si
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| 67 | pop bp
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| 68 | ret
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