;*********************************************************************
; Output Test: 8255 Centurion Laser Eyes 
;              PPI Port B output only.
;*********************************************************************

; We'll use the slow clock circuit. The crystal oscillator/resonator is just too fast at 1MHz
; to see the LEDs strobe back and forth.
; The PPI ports will be assigned addresses starting with the base address of $08 given to the PPI.

PPIbase:.EQU	$08			; UART = $00, PPI = $08, next1 = $10, next2 = $18.

PortA:	.EQU PPIbase + 0	; ($08) Port A address.
PortB:	.EQU PPIbase + 1	; ($09) Port B address.
PortC:	.EQU PPIbase + 2	; ($0A) Port C address.
CWR:	.EQU PPIbase + 3	; ($0B) Control Word Register.
CWRCfg:	.EQU $80		    ; Control word: 1000 0000
							;   Active, Mode 0, A & B & C outputs.
							; Bit D7: Mode is active. 
							; Bits D6 to D3: Group A.
							;   Bits D6  & D5: Mode 0: basic input/output.
							;   Bit D4: Port A output
							;   Bit D3: Port C upper output.
							; Bits D2 to D0: Group B.
							;	Bit D2: Port C lower output.
							;	Bit D1: Port B output.
							; 	Bit D0:	Port C lower output.
.ORG	$0000
		
		NOP             ; Safety filler NOP.
		LD A,	CWRCfg	; Set CWR so all 3 ports are output.
		OUT	(CWR),	A 	; Send control word 0x80 to CWR.

        LD A,	1       ; D0 bit selected.

Left:   OUT	(PortB), A  ; Output the bit on the databus to the 8255 port B.
        RLCA            ; Shift the bit left to the next bit position.
        CP	128			; Has the bit rotated left to bit position 7?
        JR Z,	Right   ; If yes, change direction by going to Right.
        JR	Left        ; Else no, keep rotating Left.

Right:  OUT (PortB), A  ; Output the bit on the databus to the 8255 port B. 
        RRCA            ; Shift the bit right to the next bit position.
        CP 1            ; Has the bit rotated right to bit position 0?
        JR Z,	Left    ; If yes, change direction by going to Left.
        JR	Right       ; Else no, keep rotating Right.
.END