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243 lines
9.0 KiB
243 lines
9.0 KiB
title "Compute Checksum"
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;
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; Copyright (c) 1985-2000 Microsoft Corporation
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; This file is part of the Microsoft Research IPv6 Network Protocol Stack.
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; You should have received a copy of the Microsoft End-User License Agreement
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; for this software along with this release; see the file "license.txt".
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; If not, please see http://www.research.microsoft.com/msripv6/license.htm,
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; or write to Microsoft Research, One Microsoft Way, Redmond, WA 98052-6399.
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;
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; Abstract:
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;
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; This module implements a function to compute the checksum of a buffer.
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;
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; Environment:
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;
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; Any mode.
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;
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LOOP_UNROLLING_BITS equ 5
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LOOP_UNROLLING equ (1 SHL LOOP_UNROLLING_BITS)
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.386
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.model small,c
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assume cs:FLAT,ds:FLAT,es:FLAT,ss:FLAT
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assume fs:nothing,gs:nothing
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.xlist
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include callconv.inc
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.list
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.code
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;++
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;
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; ULONG
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; tcpxsum(
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; IN ULONG cksum,
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; IN PUCHAR buf,
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; IN ULONG len
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; )
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;
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; Routine Description:
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;
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; This function computes the checksum of the specified buffer.
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;
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; Arguments:
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;
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; cksum - Suppiles the initial checksum value, in 16-bit form,
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; with the high word set to 0.
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;
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; buf - Supplies a pointer to the buffer to the checksum buffer.
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;
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; len - Supplies the length of the buffer in bytes.
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;
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; Return Value:
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;
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; The computed checksum in 32-bit two-partial-accumulators form, added to
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; the initial checksum, is returned as the function value.
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;
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;--
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cksum equ 12 ; stack offset to initial checksum
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buf equ 16 ; stack offset to source address
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len equ 20 ; stack offset to length in words
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to_checksum_last_word:
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jmp checksum_last_word
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to_checksum_done:
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jmp checksum_done
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to_checksum_dword_loop_done:
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jmp checksum_dword_loop_done
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cPublicProc tcpxsum,3
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push ebx ; save nonvolatile register
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push esi ; save nonvolatile register
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mov ecx,[esp + len] ; get length in bytes
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sub eax,eax ; clear computed checksum
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test ecx,ecx ; any bytes to checksum at all?
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jz short to_checksum_done ; no bytes to checksum
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;
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; if the checksum buffer is not word aligned, then add the first byte of
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; the buffer to the input checksum.
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;
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mov esi,[esp + buf] ; get source address
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sub edx,edx ; set up to load word into EDX below
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test esi,1 ; check if buffer word aligned
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jz short checksum_word_aligned ; if zf, buffer word aligned
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mov ah,[esi] ; get first byte (we know we'll have
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; to swap at the end)
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inc esi ; increment buffer address
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dec ecx ; decrement number of bytes
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jz short to_checksum_done ; if zf set, no more bytes
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;
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; If the buffer is not an even number of of bytes, then initialize
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; the computed checksum with the last byte of the buffer.
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;
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checksum_word_aligned: ;
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shr ecx,1 ; convert to word count
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jnc short checksum_start ; if nc, even number of bytes
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mov al,[esi+ecx*2] ; initialize the computed checksum
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jz short to_checksum_done ; if zf set, no more bytes
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;
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; Compute checksum in large blocks of dwords, with one partial word up front if
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; necessary to get dword alignment, and another partial word at the end if
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; needed.
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;
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;
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; Compute checksum on the leading word, if that's necessary to get dword
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; alignment.
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;
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checksum_start: ;
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test esi,02h ; check if source dword aligned
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jz short checksum_dword_aligned ; source is already dword aligned
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mov dx,[esi] ; get first word to checksum
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add esi,2 ; update source address
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add eax,edx ; update partial checksum
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; (no carry is possible, because EAX
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; and EDX are both 16-bit values)
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dec ecx ; count off this word (zero case gets
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; picked up below)
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;
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; Checksum as many words as possible by processing a dword at a time.
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;
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checksum_dword_aligned:
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push ecx ; so we can tell if there's a trailing
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; word later
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shr ecx,1 ; # of dwords to checksum
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jz short to_checksum_last_word ; no dwords to checksum
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mov edx,[esi] ; preload the first dword
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add esi,4 ; point to the next dword
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dec ecx ; count off the dword we just loaded
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jz short to_checksum_dword_loop_done
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; skip the loop if that was the only
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; dword
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mov ebx,ecx ; EBX = # of dwords left to checksum
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add ecx,LOOP_UNROLLING-1 ; round up loop count
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shr ecx,LOOP_UNROLLING_BITS ; convert from word count to unrolled
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; loop count
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and ebx,LOOP_UNROLLING-1 ; # of partial dwords to do in first
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; loop
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jz short checksum_dword_loop ; special-case when no partial loop,
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; because fixup below doesn't work
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; in that case (carry flag is
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; cleared at this point, as required
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; at loop entry)
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lea esi,[esi+ebx*4-(LOOP_UNROLLING*4)]
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; adjust buffer pointer back to
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; compensate for hardwired displacement
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; at loop entry point
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; ***doesn't change carry flag***
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jmp loop_entry[ebx*4] ; enter the loop to do the first,
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; partial iteration, after which we can
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; just do 64-word blocks
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; ***doesn't change carry flag***
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checksum_dword_loop:
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DEFLAB macro pre,suf
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pre&suf:
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endm
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TEMP=0
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REPT LOOP_UNROLLING
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deflab loop_entry_,%TEMP
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adc eax,edx
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mov edx,[esi + TEMP]
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TEMP=TEMP+4
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ENDM
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checksum_dword_loop_end:
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lea esi,[esi + LOOP_UNROLLING * 4] ; update source address
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; ***doesn't change carry flag***
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dec ecx ; count off unrolled loop iteration
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; ***doesn't change carry flag***
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jnz checksum_dword_loop ; do more blocks
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checksum_dword_loop_done label proc
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adc eax,edx ; finish dword checksum
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mov edx,0 ; prepare to load trailing word
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adc eax,edx
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;
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; Compute checksum on the trailing word, if there is one.
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; High word of EDX = 0 at this point
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; Carry flag set iff there's a trailing word to do at this point
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;
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checksum_last_word label proc ; "proc" so not scoped to function
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pop ecx ; get back word count
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test ecx,1 ; is there a trailing word?
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jz short checksum_done ; no trailing word
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add ax,[esi] ; add in the trailing word
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adc eax,0 ;
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checksum_done label proc ; "proc" so not scoped to function
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mov ecx,eax ; fold the checksum to 16 bits
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ror ecx,16
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add eax,ecx
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mov ebx,[esp + buf]
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shr eax,16
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test ebx,1 ; check if buffer word aligned
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jz short checksum_combine ; if zf set, buffer word aligned
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ror ax,8 ; byte aligned--swap bytes back
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checksum_combine label proc ; "proc" so not scoped to function
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add ax,word ptr [esp + cksum] ; combine checksums
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pop esi ; restore nonvolatile register
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adc eax,0 ;
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pop ebx ; restore nonvolatile register
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stdRET tcpxsum
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REFLAB macro pre,suf
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dd pre&suf
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endm
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align 4
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loop_entry label dword
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dd 0
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TEMP=LOOP_UNROLLING*4
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REPT LOOP_UNROLLING-1
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TEMP=TEMP-4
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reflab loop_entry_,%TEMP
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ENDM
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stdENDP tcpxsum
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end
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