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; Test VIA's PORTB using LEDs in Cylon pattern.
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	processor 6502	; Processor directive for Assembler.

PORTB = $6000	; Our data port (Port B).
PORTA = $6001	; Our control port (Port A).
DDRB =  $6002	; Data Direction Register for Port B. 1: output, 0: input.
DDRA =  $6003	; Data Direction Register for Port A.
Reset = $8000	; Reset vector address of $8000 (start of ROM).

	;seg .U code
	org $8000	; Define the code origin of $8000 (ROM) starting point.
	
Start:
	lda	#$FF	; A9 FF		; All pins will be outputs.
	sta	$6002	; 8D 02 60	; DDRB: Data Direction Register port B.	
	lda #$00	; A9 00		; Blank the LEDs.
	sta $6000	; 8D 00 60	; Write to port B (PORTB).
	lda	#$01	; A9 01		; Light only the first LED, the low order bit 1.	
	sta $6000	; 8D 00 60	; Write to data port B (PORTB).	
R2L:	
	rol	        ; 2A		; Rotate bit one position to the left.
	sta	$6000	; 8D 00 60	; Write to data port B (PORTB).	
	cmp #$80	; C9 80
	beq L2Ra	; F0 03		; Relative branch if equal to zero.
	jmp R2L		; 4C 0F 80	; Repeat indefinitely by jumping back to $800F.

L2Ra:
	sta $6000	; 8D 00 60
L2Rb:
	lsr			; 4A		; Logical shift right
	sta $6000	; 8D 00 60	; Display the result.
	cmp #$00	; C9 00		; Is this the 0/1 BIT?
	beq R2L		; F0 EA		; Yes? Shift-right routine is finished.
	jmp L2Rb	; 4C 1D 80	; No? Continue shifting bit to the right.
	
	org	$FFFC
	word $8000	; Reset vector is set to location $8000.
	word $0000	; Fill 32K to end.