====== Scanning the keyboard without joysticks interfere ======

Converts keycode ($00-$3F) to $DC00-$DC01 values and scans without interfering from joysticks at port 1 or 2. Also keeps history, to detect key press & hold.

<code>

;64tass format
;with default $dc02 - $dc03 values ($ff and $00)
;taken from Platoon game, shortened and key history integrated.
;code to key conversion table below, from c64-wiki.com

;to use inside irq, below code is enough :
;	lda #$keycode
;	jsr keyscan
;	bcc +
;	inc $d020
;+	rts 

;$00	Inst/Del	$10	5	$20	9	$30	£
;$01	Return		$11	R	$21	I	$31	*
;$02	Crsr ←→		$12	D	$22	J	$32	]
;$03	F7/F8		$13	6	$23	0	$33	Clr/Home
;$04	F1/F2		$14	C	$24	M	$34	R.Shift
;$05	F3/F4		$15	F	$25	K	$35	=
;$06	F5/F6		$16	T	$26	O	$36	↑
;$07	Crsr ↑↓		$17	X	$27	N	$37	?
;$08	3		$18	7	$28	+	$38	1
;$09	W		$19	Y	$29	P	$39	←
;$0A	A		$1A	G	$2A	L	$3A	Ctrl
;$0B	4		$1B	8	$2B	−	$3B	2
;$0C	Z		$1C	B	$2C	>	$3C	Space
;$0D	S		$1D	H	$2D	[	$3D	Commodore
;$0E	E		$1E	U	$2E	@	$3E	Q
;$0F	L.Shift		$1F	V	$2F	<	$3F	Run/Stop


	* = $1000
		
	lda #$3e		;scan for key "Q"
	jsr keyscan
	bcc +
	inc $d020		;C=1, this code executed if key is pressed
+		
-	lda $d011
	bpl -
-	lda $d011
	bmi -
		
	jmp $1000	
		
keyscan	
;	sty backy+1		;remove comment if you need .Y preserved
	pha
	lsr
	lsr
	lsr
	tay
	lda row,y
	sta $dc00
	pla
	and #$07
	tay
	lda $dc01
	and column,y
	bne nokey
	lda #$ff		;key is checked 2nd time, to be sure of
	sta $dc00		;joy #1 and #2 are not interfering
	lda $dc01
	and column,y
	beq nokey
		
	cmp histv		;history value ?
	bne +								
	clc		        ;case #3, key is held down. keep history & set C=0
	bcc backy
		
nokey	asl			;case #1, no keypress, update history value 2x & set C=0
	clc									
	.byte $24							
+	sec			;case #2, key pressed&released. update history & set C=1
	sta histv
		
backy	
;	ldy #$00		;remove comment if you need .Y preserved
	lda #$ff
	sta $dc00		;set default value
	lda #$7f
	sta $dc01		;set default value
	rts

		
row	.byte $fe,$fd,$fb,$f7,$ef,$df,$bf,$7f
column	.byte $01,$02,$04,$08,$10,$20,$40,$80		
histv	.byte $00

</code>