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title "Compute Checksum"
;/*++;; Copyright (c) 1992 Microsoft Corporation;; Module Name:;; chksum.asm;; Abstract:;; This module implements a fucntion to compute the checksum of a buffer.;; Author:;; David N. Cutler (davec) 27-Jan-1992;; Environment:;; Any mode.;; Revision History:;;--*/
.386 .model small,c
assume cs:FLAT,ds:FLAT,es:FLAT,ss:FLAT assume fs:nothing,gs:nothing
.xlist include callconv.inc include ks386.inc .list
.code
;++;; USHORT; ChkSum(; IN ULONG cksum,; IN PUSHORT buf,; IN ULONG len; );; Routine Description:;; This function computes the checksum of the specified buffer.;; Arguments:;; cksum - Suppiles the initial checksum value.;; buf - Supplies a pointer to the buffer that is checksumed.;; len - Supplies the of the buffer in words.;; Return Value:;; The computed checksum is returned as the function value.;;--
cksum equ 8 ; stack offset to initial checksumbuf equ 12 ; stack offset to source addresslen equ 16 ; stack offset to length in words
cPublicProc ChkSum,3
push esi ; save nonvolatile register mov ecx,[esp + len] ; get length in words mov esi,[esp + buf] ; get source address mov eax,[esp + cksum] ; get initial checksum shl ecx,1 ; convert to length in bytes jz cks80 ; if z set, no words to checksum
;; Compute checksum in cascading order of block size until 128 byte blocks; are all that is left, then loop on 128-bute blocks.;
test esi,02h ; check if source dword aligned jz short cks10 ; if z set, source is dword aligned sub edx,edx ; get initial word for alignment mov dx,[esi + 0] ; add eax,edx ; update partial checkcum adc eax,0 ; add carry add esi,2 ; update source address sub ecx,2 ; reduce length in bytescks10: mov edx,ecx ; isolate residual bytes and edx,07h ; sub ecx,edx ; subtract residual bytes jz cks60 ; if z set, no 8-byte blocks test ecx,08h ; test if initial 8-byte block jz short cks20 ; if z set, no initial 8-byte block add eax,[esi + 0] ; compute 8-byte checksum adc eax,[esi + 4] ; adc eax,0 ; add carry add esi,8 ; update source address sub ecx,8 ; reduce length of checksum jz cks60 ; if z set, end of 8-byte blockscks20: test ecx,010h ; test if initial 16-byte block jz short cks30 ; if z set, no initial 16-byte block add eax,[esi + 0] ; compute 16-byte checksum adc eax,[esi + 4] ; adc eax,[esi + 8] ; adc eax,[esi + 12] ; adc eax,0 ; add carry add esi,16 ; update source address sub ecx,16 ; reduce length of checksum jz cks60 ; if z set, end of 8-byte blockscks30: test ecx,020h ; test if initial 32-byte block jz short cks40 ; if z set, no initial 32-byte block add eax,[esi + 0] ; compute 32-byte checksum adc eax,[esi + 4] ; adc eax,[esi + 8] ; adc eax,[esi + 12] ; adc eax,[esi + 16] ; adc eax,[esi + 20] ; adc eax,[esi + 24] ; adc eax,[esi + 28] ; adc eax,0 ; add carry add esi,32 ; update source address sub ecx,32 ; reduce length of checksum jz cks60 ; if z set, end of 8-byte blockscks40: test ecx,040h ; test if initial 64-byte block jz cks50 ; if z set, no initial 64-byte block add eax,[esi + 0] ; compute 64-byte checksum adc eax,[esi + 4] ; adc eax,[esi + 8] ; adc eax,[esi + 12] ; adc eax,[esi + 16] ; adc eax,[esi + 20] ; adc eax,[esi + 24] ; adc eax,[esi + 28] ; adc eax,[esi + 32] ; adc eax,[esi + 36] ; adc eax,[esi + 40] ; adc eax,[esi + 44] ; adc eax,[esi + 48] ; adc eax,[esi + 52] ; adc eax,[esi + 56] ; adc eax,[esi + 60] ; adc eax,0 ; add carry add esi,64 ; update source address sub ecx,64 ; reduce length of checksum jz short cks60 ; if z set, end of 8-byte blockscks50: add eax,[esi + 0] ; compute 64-byte checksum adc eax,[esi + 4] ; adc eax,[esi + 8] ; adc eax,[esi + 12] ; adc eax,[esi + 16] ; adc eax,[esi + 20] ; adc eax,[esi + 24] ; adc eax,[esi + 28] ; adc eax,[esi + 32] ; adc eax,[esi + 36] ; adc eax,[esi + 40] ; adc eax,[esi + 44] ; adc eax,[esi + 48] ; adc eax,[esi + 52] ; adc eax,[esi + 56] ; adc eax,[esi + 60] ; adc eax,[esi + 64] ; adc eax,[esi + 68] ; adc eax,[esi + 72] ; adc eax,[esi + 76] ; adc eax,[esi + 80] ; adc eax,[esi + 84] ; adc eax,[esi + 88] ; adc eax,[esi + 92] ; adc eax,[esi + 96] ; adc eax,[esi + 100] ; adc eax,[esi + 104] ; adc eax,[esi + 108] ; adc eax,[esi + 112] ; adc eax,[esi + 116] ; adc eax,[esi + 120] ; adc eax,[esi + 124] ; adc eax,0 ; add carry add esi,128 ; update source address sub ecx,128 ; reduce length of checksum jnz short cks50 ; if z clear, not end of 8-byte blocks
;; Compute checksum on 2-byte blocks.;
cks60: test edx,edx ; check if any 2-byte blocks jz short cks80 ; if z set, no 2-byte blockscks70: sub ecx,ecx ; load 2-byte block mov cx,[esi + 0] ; add eax,ecx ; compue 2-byte checksum adc eax,0 ; add esi,2 ; update source address sub edx,2 ; reduce length of checksum jnz short cks70 ; if z clear, more 2-bytes blocks
;; Fold 32-but checksum into 16-bits;
cks80: mov edx,eax ; copy checksum value shr edx,16 ; isolate high order bits and eax,0ffffh ; isolate low order bits add eax,edx ; sum high and low order bits mov edx,eax ; isolate possible carry shr edx,16 ; add eax,edx ; add carry and eax,0ffffh ; clear possible carry bit pop esi ; restore nonvolatile register stdRET ChkSum
stdENDP ChkSum
end
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