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

000000r 1               ;
000000r 1               ; ACORN System 1 Applications: Single 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               QHEXTD1	:=		$FE64		; quad hex to display 1
000000r 1               QDATFET	:=		$FE88		; quad data fetch
000000r 1               RESTART	:=		$FF04		; Monitor renentry point
000000r 1               ;
000000r 1               		.org			$0200
000200  1               ;
000200  1  D8           SMULT:	cld
000201  1  EA           		nop					; serves no purpose ?
000202  1  84 20        		sty		$20			; setup prompt for zero multiplier
000204  1  A9 11        		lda		#$11
000206  1  85 21        		sta		$21			; prompt for first - multiplicand
000208  1  A2 20        		ldx		#$20
00020A  1  20 88 FE     		jsr		QDATFET		; fetch the numbers
00020D  1  98           		tya					; clears A
00020E  1  A0 08        		ldy		#$08		; loop counter
000210  1  66 20        LOOP:	ror		$20			; shift multiplier (and high byte of result)
000212  1  90 03        		bcc		NADS		; no add if not bit
000214  1  18           		clc
000215  1  65 21        		adc		$21			; add multiplicand into low byte of result
000217  1  6A           NADS:	ror		A			; and shift low byte of result
000218  1  88           		dey
000219  1  D0 F5        		bne		LOOP
00021B  1  85 21        		sta		$21			; put in low byte
00021D  1  66 20        		ror		$20			; final justification shift
00021F  1  20 64 FE     		jsr		QHEXTD1		; display answer
000222  1  4C 04 FF     		jmp		RESTART
000222  1