|
|
title "Compute Checksum";/*++;; Copyright (c) 2001 Microsoft Corporation;; Module Name:;; chksum.asm;; Abstract:;; This module implements a fucntion to compute the checksum of a buffer.;; Author:;; David N. Cutler (davec) 25-Jan-2001;; Environment:;; Any mode.;; Revision History:;;--*/
include ksamd64.inc
subttl "Checksum";++;; USHORT; ChkSum(; IN ULONG cksum,; IN PUSHORT buf,; IN ULONG len; );; Routine Description:;; This function computes the checksum of the specified buffer.;; Arguments:;; cksum (ecx) - Suppiles the initial checksum value.;; buf (rdx) - Supplies a pointer to the buffer that is checksumed.;; len (r8d) - Supplies the of the buffer in words.;; Return Value:;; The computed checksum is returned as the function value.;;--
LEAF_ENTRY ChkSum, _TEXT$00
mov eax, ecx ; set initial checksum value mov ecx, r8d ; set length of buffer in words shl ecx, 1 ; convert to length in bytes jz cks80 ; if z, 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-byte blocks.;
test rdx, 02h ; check if source dword aligned jz short cks10 ; if z, source is dword aligned xor r8, r8 ; get initial word for alignment mov r8w, [rdx] ; add eax, r8d ; update partial checkcum adc eax, 0 ; add carry add rdx, 2 ; update source address sub ecx, 2 ; reduce length in bytescks10: mov r8d, ecx ; isolate residual bytes and r8d, 07h ; sub ecx, r8d ; subtract residual bytes jz cks60 ; if z, no 8-byte blocks test ecx, 08h ; test if initial 8-byte block jz short cks20 ; if z, no initial 8-byte block add eax, [rdx] ; compute 8-byte checksum adc eax, 4[rdx] ; adc eax, 0 ; add carry add rdx, 8 ; update source address sub ecx, 8 ; reduce length of checksum jz cks60 ; if z, end of 8-byte blockscks20: test ecx, 010h ; test if initial 16-byte block jz short cks30 ; if z, no initial 16-byte block add eax, [rdx] ; compute 16-byte checksum adc eax, 4[rdx] ; adc eax, 8[rdx] ; adc eax, 12[rdx] ; adc eax, 0 ; add carry add rdx, 16 ; update source address sub ecx, 16 ; reduce length of checksum jz cks60 ; if z, 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, [rdx] ; compute 32-byte checksum adc eax, 4[rdx] ; adc eax, 8[rdx] ; adc eax, 12[rdx] ; adc eax, 16[rdx] ; adc eax, 20[rdx] ; adc eax, 24[rdx] ; adc eax, 28[rdx] ; adc eax, 0 ; add carry add rdx, 32 ; update source address sub ecx, 32 ; reduce length of checksum jz cks60 ; if z, end of 8-byte blockscks40: test ecx, 040h ; test if initial 64-byte block jz cks50 ; if z, no initial 64-byte block add eax, [rdx] ; compute 64-byte checksum adc eax, 4[rdx] ; adc eax, 8[rdx] ; adc eax, 12[rdx] ; adc eax, 16[rdx] ; adc eax, 20[rdx] ; adc eax, 24[rdx] ; adc eax, 28[rdx] ; adc eax, 32[rdx] ; adc eax, 36[rdx] ; adc eax, 40[rdx] ; adc eax, 44[rdx] ; adc eax, 48[rdx] ; adc eax, 52[rdx] ; adc eax, 56[rdx] ; adc eax, 60[rdx] ; adc eax, 0 ; add carry add rdx, 64 ; update source address sub ecx, 64 ; reduce length of checksum jz cks60 ; if z, end of 8-byte blockscks50: add eax, [rdx] ; compute 128-byte checksum adc eax, 4[rdx] ; adc eax, 8[rdx] ; adc eax, 12[rdx] ; adc eax, 16[rdx] ; adc eax, 20[rdx] ; adc eax, 24[rdx] ; adc eax, 28[rdx] ; adc eax, 32[rdx] ; adc eax, 36[rdx] ; adc eax, 40[rdx] ; adc eax, 44[rdx] ; adc eax, 48[rdx] ; adc eax, 52[rdx] ; adc eax, 56[rdx] ; adc eax, 60[rdx] ; adc eax, 64[rdx] ; adc eax, 68[rdx] ; adc eax, 72[rdx] ; adc eax, 76[rdx] ; adc eax, 80[rdx] ; adc eax, 84[rdx] ; adc eax, 88[rdx] ; adc eax, 92[rdx] ; adc eax, 96[rdx] ; adc eax, 100[rdx] ; adc eax, 104[rdx] ; adc eax, 108[rdx] ; adc eax, 112[rdx] ; adc eax, 116[rdx] ; adc eax, 120[rdx] ; adc eax, 124[rdx] ; adc eax, 0 ; add carry add rdx, 128 ; update source address sub ecx, 128 ; reduce length of checksum jnz short cks50 ; if nz, not end of 8-byte blocks
;; Compute checksum on 2-byte blocks.;
cks60: test r8d, r8d ; check if any 2-byte blocks jz short cks80 ; if z, no 2-byte blocks xor ecx, ecx ; clear entire registercks70: mov cx, [rdx] ; load 2-byte block add eax, ecx ; compute 2-byte checksum adc eax, 0 ; add rdx, 2 ; update source address sub r8d, 2 ; reduce length of checksum jnz short cks70 ; if nz, 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 ret ;
LEAF_END ChkSum, _TEXT$00
end
|
