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INCHKSUM.ASM
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Assembly Source File
|
1994-11-28
|
1KB
|
62 lines
;
;
; Usage :
; void far *inchksum()
;
; Internet compatible 1's complement checksum
;
; (c) 1990 University of Waterloo,
; Faculty of Engineering,
; Engineering Microcomputer Network Development Office
;
; version
;
; 0.1 17 Dec -1990 E. P. Engelke
;
;
include masmdefs.hsm
include model.hsm
codedef INCHKSUM
datadef
cstart INCHKSUM
cpublic inchksum
; Compute 1's-complement sum of data buffer
;
; unsigned lcsum( usigned far *buf, unsigned cnt)
push DS
lds SI, +@AB + 0 [BP]
mov CX, +@AB + 4 [BP] ; cx = cnt
mov BL, CL
shr CX, 1 ; group into words
xor DX, DX ; set checksum to 0
cld
jcxz remain
clc
deloop: lodsw
adc DX, AX
loop deloop
adc DX, 0 ; only two necessary
adc DX, 0
remain: and BL, 1
jz done
xor AH, AH
lodsb
add DX, AX
adc DX, 0
adc DX, 0
done: mov AX,DX ; result into ax
or AX,AX
ok: pop DS
creturn inchksum
cend INCHKSUM
end