| 1 | ! oneC_sum() - One complement`s checksum Author: Kees J. Bot
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| 2 | ! 9 May 1995
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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 | .sect .text
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| 7 |
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| 8 | .define _oneC_sum
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| 9 | .align 16
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| 10 | _oneC_sum:
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| 11 | push ebp
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| 12 | mov ebp, esp
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| 13 | push esi
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| 14 | push edi
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| 15 | movzx eax, 8(ebp) ! Checksum of previous block
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| 16 | mov esi, 12(ebp) ! Data to compute checksum over
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| 17 | mov edi, 16(ebp) ! Number of bytes
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| 18 |
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| 19 | xor edx, edx
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| 20 | xorb cl, cl
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| 21 | align: test esi, 3 ! Is the data aligned?
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| 22 | jz aligned
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| 23 | test edi, edi
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| 24 | jz 0f
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| 25 | movb dl, (esi) ! Rotate the first unaligned bytes
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| 26 | dec edi ! into the edx register
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| 27 | 0: inc esi
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| 28 | ror edx, 8
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| 29 | ror eax, 8 ! Rotate the checksum likewise
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| 30 | addb cl, 8 ! Number of bits rotated
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| 31 | jmp align
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| 32 | aligned:add eax, edx ! Summate the unaligned bytes
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| 33 | adc eax, 0 ! Add carry back in for one`s complement
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| 34 |
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| 35 | jmp add6test
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| 36 | .align 16
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| 37 | add6: add eax, (esi) ! Six times unrolled loop, see below
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| 38 | adc eax, 4(esi)
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| 39 | adc eax, 8(esi)
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| 40 | adc eax, 12(esi)
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| 41 | adc eax, 16(esi)
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| 42 | adc eax, 20(esi)
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| 43 | adc eax, 0
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| 44 | add esi, 24
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| 45 | add6test:
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| 46 | sub edi, 24
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| 47 | jae add6
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| 48 | add edi, 24
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| 49 |
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| 50 | jmp add1test
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| 51 | .align 16
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| 52 | add1: add eax, (esi) ! while ((edi -= 4) >= 0)
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| 53 | adc eax, 0 ! eax += *esi++;
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| 54 | add esi, 4 ! edi += 4;
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| 55 | add1test:
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| 56 | sub edi, 4
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| 57 | jae add1
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| 58 | add edi, 4
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| 59 |
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| 60 | jz done ! Are there extra bytes?
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| 61 | mov edx, (esi) ! Load extra bytes in a full dword
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| 62 | and edx, mask-4(edi*4) ! Mask off excess
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| 63 | add eax, edx ! Add in the last bits
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| 64 | adc eax, 0
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| 65 | done: rol eax, cl ! Undo the rotation at the beginning
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| 66 | mov edx, eax
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| 67 | shr eax, 16
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| 68 | o16 add ax, dx ! Add the two words in eax to form
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| 69 | o16 adc ax, 0 ! a 16 bit sum
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| 70 | pop edi
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| 71 | pop esi
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| 72 | pop ebp
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| 73 | ret
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| 74 |
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| 75 | .sect .rom
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| 76 | .align 4
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| 77 | mask: .data4 0x000000FF, 0x0000FFFF, 0x00FFFFFF
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| 78 |
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| 79 | !
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| 80 | ! $PchId: oneC_sum.ack.s,v 1.2 1996/03/12 19:33:51 philip Exp $
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