.TITLE	UPCSUM - UPDATE CHECKSUM
	.IDENT	/V1.01/
	.NLIST	TOC,SYM
	.ENABL	LC

; Part of Sira Paper Tape Punch Despooler.
;
; COPYRIGHT(C) 1981,1982 Sira Institute Ltd.,
;	South Hill, Chislehurst, Kent, BR7 5EH, England.
;
; Author: C.J. Doran
;
; Assemble as:
;	>MAC PPPUPCSUM=PPPUPCSUM
;+
; Add bits of character in %0 to appropriate bytes of checksum.
;-
	.PSECT	CODE,I,RO

UPCSUM::MOV	%1,-(SP)	; Save %1
	MOVB	%0,-(SP)	; Push character
	MOV	#CHKSUM+8.,%1	; Address end of checksum block
10$:	TSTB	@SP		; See if any bits left (clearing carry)
	BEQ	20$		; No more, exit
	RORB	@SP		; Shift out a bit
	ADCB	-(%1)		; Add to appropriate checksum byte
	BR	10$		; Repeat
20$:	TST	(SP)+		; Done, purge stack,
	MOV	(SP)+,%1	; Restore register
	RTS	PC		; and return

	.END