80 lines
1.7 KiB
ArmAsm
80 lines
1.7 KiB
ArmAsm
! oneC_sum() - One complement`s checksum Author: Kees J. Bot
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! 9 May 1995
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! See RFC 1071, "Computing the Internet checksum"
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! See also the C version of this code.
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.sect .text
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.define _oneC_sum
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.align 16
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_oneC_sum:
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push ebp
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mov ebp, esp
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push esi
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push edi
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movzx eax, 8(ebp) ! Checksum of previous block
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mov esi, 12(ebp) ! Data to compute checksum over
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mov edi, 16(ebp) ! Number of bytes
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xor edx, edx
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xorb cl, cl
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align: test esi, 3 ! Is the data aligned?
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jz aligned
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test edi, edi
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jz 0f
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movb dl, (esi) ! Rotate the first unaligned bytes
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dec edi ! into the edx register
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0: inc esi
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ror edx, 8
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ror eax, 8 ! Rotate the checksum likewise
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addb cl, 8 ! Number of bits rotated
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jmp align
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aligned:add eax, edx ! Summate the unaligned bytes
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adc eax, 0 ! Add carry back in for one`s complement
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jmp add6test
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.align 16
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add6: add eax, (esi) ! Six times unrolled loop, see below
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adc eax, 4(esi)
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adc eax, 8(esi)
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adc eax, 12(esi)
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adc eax, 16(esi)
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adc eax, 20(esi)
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adc eax, 0
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add esi, 24
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add6test:
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sub edi, 24
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jae add6
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add edi, 24
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jmp add1test
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.align 16
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add1: add eax, (esi) ! while ((edi -= 4) >= 0)
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adc eax, 0 ! eax += *esi++;
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add esi, 4 ! edi += 4;
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add1test:
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sub edi, 4
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jae add1
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add edi, 4
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jz done ! Are there extra bytes?
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mov edx, (esi) ! Load extra bytes in a full dword
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and edx, mask-4(edi*4) ! Mask off excess
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add eax, edx ! Add in the last bits
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adc eax, 0
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done: rol eax, cl ! Undo the rotation at the beginning
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mov edx, eax
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shr eax, 16
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o16 add ax, dx ! Add the two words in eax to form
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o16 adc ax, 0 ! a 16 bit sum
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pop edi
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pop esi
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pop ebp
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ret
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.sect .rom
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.align 4
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mask: .data4 0x000000FF, 0x0000FFFF, 0x00FFFFFF
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!
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! $PchId: oneC_sum.ack.s,v 1.2 1996/03/12 19:33:51 philip Exp $
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