ca65 V2.17 - Git 582aa41
Main file   : double_byte_multiply.ca65
Current file: double_byte_multiply.ca65

000000r 1               ;
000000r 1               ; ACORN System 1 Applications: Double Byte Multiply
000000r 1               ;
000000r 1               ; From the Acorn System 1 User's Manual
000000r 1               ;
000000r 1               ; (Chris Oddy January 2022)
000000r 1               ;
000000r 1               ; entry point $0200
000000r 1               ;
000000r 1               		.setcpu			"6502"
000000r 1               		.listbytes		unlimited
000000r 1               ;
000000r 1               MPIER	:=		$20
000000r 1               MPICAND	:=		$22
000000r 1               QHEXTD2	:=		$FE66		; quad hex to display 2
000000r 1               QDATFET	:=		$FE88		; quad data fetch
000000r 1               RESTART	:=		$FF04		; Monitor reentry point
000000r 1               ;
000000r 1               		.org			$0200
000200  1               ;
000200  1  D8           DMULT:	cld					; binary only
000201  1  84 20        		sty		MPIER		; form prompt for the zero input
000203  1  84 21        		sty		MPIER+1
000205  1  A9 11        		lda		#$11
000207  1  85 22        		sta		MPICAND		; form prompt for the first input
000209  1  85 23        		sta		MPICAND+1
00020B  1  A2 20        		ldx		#$20
00020D  1  20 88 FE     		jsr		QDATFET		; fetch zero input
000210  1  A2 22        		ldx		#$22
000212  1  20 88 FE     		jsr		QDATFET
000215  1  84 24        		sty		$24			; clear working space
000217  1  84 25        		sty		$25
000219  1  A0 10        		ldy		#$10		; loop count initialisation
00021B  1  66 23        LOOP:	ror		MPICAND+1	; two byte shift right
00021D  1  66 22        		ror		MPICAND
00021F  1  90 0D        		bcc		NADD		; no add if the o/p bit isn't a one
000221  1  18           		clc
000222  1  A5 20        		lda		MPIER		; two byte add
000224  1  65 24        		adc		$24
000226  1  85 24        		sta		$24
000228  1  A5 21        		lda		MPIER+1
00022A  1  65 25        		adc		$25
00022C  1  85 25        		sta		$25			; no carry out of the add
00022E  1  66 25        NADD:	ror		$25			; shift again
000230  1  66 24        		ror		$24
000232  1  88           		dey
000233  1  D0 E6        		bne		LOOP		; go round loop 16 times
000235  1  66 23        		ror		MPICAND+1
000237  1  66 22        		ror		MPICAND
000239  1  A0 06        		ldy		#$06		; set up position
00023B  1  20 66 FE     		jsr		QHEXTD2		; X already pointing at correct locations
00023E  1               							; put 4 hex out
00023E  1  A0 02        		ldy		#$02		; next position
000240  1  A2 24        		ldx		#$24		; set up X
000242  1  20 66 FE     		jsr		QHEXTD2		; put next 4 out
000245  1  4C 04 FF     		jmp		RESTART		; display result
000245  1