.org $c000

VIDEO = $B000
JOYSTICK = $B001
COUNTER = $9FFF
DDRB1 = $B002
DDRA1 = $B003
PCR1 = $B00C
IFR1 = $B00D
IER1 = $B00E
KBIN = $B010
KBOUT = $B011
DDRB2 = $B012
DDRA2 = $B013
PCR2 = $B01C
IFR2 = $B01D
IER2 = $B01E
LCR = $B023
FCR = $B022
USTAT = $B025

sysinit:

	JSR viainit
	JSR videoinit
	JSR irqinit
	JSR videotest
	JSR uartinit
	JMP system

system:

	JSR kbinput

viainit:

	LDA #$FF
	STA DDRB1	; Output to MC6847
	LDA #$00
	STA DDRA1	; Input from joystick
	LDA #$00
	STA DDRB2	; Input from keyboard
	LDA #$FF
	STA DDRA2	; Output to keyboard
	LDA #$FF
	STA KBOUT
	RTS

videoinit:

	LDA #$00
	STA VIDEO	; set all control bits of MC6847 to low
	LDA #$00
	STA COUNTER	; set video counter to zero
	RTS

irqinit:
	
	LDA #%10000010 ; Enable interrupt on CA1
	STA IER1
	LDA #%00000000 ; Set CA1 Negative Active Edge
	STA PCR1
	LDA #%10010000 ; Enable interrupt on CB1
	STA IER2
	LDA #%00010000 ; Set CB1 Positive Active Edge
	STA PCR2
	RTS

uartinit:
	LDA #%10000011 ; enable divisor latch, send break off, no parity, 1 stop bit, 8 bit data
	STA LCR ; store to line controller register
	LDA #$60 ; divisor as decimal 96 to set 1200 baud
	STA $B020 ; LSB of divisor latch
	LDA #$00
	STA $B021 ; MSB of divisor latch
	LDA #%00000011 ; disable divisor latch
	STA LCR
	LDA #%11100000 ; enable FIFO, reset everything, FIFO trigger level 1 byte (may need adjusting)
	STA FCR
	RTS

uartstat:
	LDA USTAT
	TXS
	LDX COUNTER
	STA $8000,X
	TSX
	RTS

videotest:

	LDA #$13
	STA $8000	; print T
	JSR videocount
	LDA #$04
	STA $8001	; print E
	JSR videocount
	LDA #$12
	STA $8002	; print S
	JSR videocount
	LDA #$13
	STA $8003	; print T
	JSR videocount
	RTS

videocount:
	LDX COUNTER	; load value of video counter to X
	INX		; increment video counter
	STX COUNTER 	; store X to video counter
	RTS

kbinput:
	LDA #%00000001
	JSR keysub
	BNE kb1
	LDA #%00000010
	JSR keysub
	BNE kb2
	LDA #%00000100
	JSR keysub
	BNE kb3
	LDA #%00001000
	JSR keysub
	BNE kb4
	LDA #%00010000
	JSR keysub
	BNE kb5
	LDA #%00100000
	JSR keysub
	BNE kb6
	LDA #%01000000
	JSR keysub
	BNE kb7
	LDA #%10000000
	JSR keysub
	BNE kb8
	PLA
	RTI

kb1:
kb2:
kb3:
kb4:
kb5:
kb6:
kb7:
kb8:

keysub:

	STA KBOUT
	LDA KBIN
	RTS

nmi:

	RTI

irq:

	PHA
	LDA IFR1
	CMP #%10000010
	BEQ YAMAHA
	LDA IFR2
	CMP #%10010000
	BEQ kbinput
	PLA
	RTI

	.org $fffa
	.word nmi
	.word sysinit
	.word irq