;
;
;  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
	push	SI
	push	BX
        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

; resolve remainder
;loop2:	adc     DX, 0
;	jc      loop2
        adc     DX, 0                   ; only two necessary
        adc     DX, 0

remain: and     BL, 1
	jz	done
;
        xor     AH, AH

	lodsb
;	clc
;	add     DX, AX
;loop3:	adc     DX, 0
;	jc      loop3
        add     DX, AX
	adc     DX, 0
	adc     DX, 0

done:   mov	AX,DX		; result into ax
        or      AX,AX
;	jnz     ok
;	xchg	AL,AH		; byte swap result (8088 is little-endian)
;	mov     AX, 0ffffh
ok:	pop	BX
	pop	SI
	pop     DS
creturn inchksum
cend    INCHKSUM
        end