From 57b380829d33bf8bd37d4f4954ab8ffe001c5795 Mon Sep 17 00:00:00 2001
From: Paul Warren <pwarren@pwarren.id.au>
Date: Sun, 2 Aug 2020 22:46:31 +1000
Subject: [PATCH] Interreupt handling

---
 interrupt.s | 187 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 187 insertions(+)
 create mode 100644 interrupt.s

diff --git a/interrupt.s b/interrupt.s
new file mode 100644
index 0000000..298656f
--- /dev/null
+++ b/interrupt.s
@@ -0,0 +1,187 @@
+PORTB = $6000
+PORTA = $6001
+DDRB = $6002
+DDRA = $6003
+
+value = $0200 			; 2 bytes
+mod10 = $0202			; 2 bytes
+message = $0204 		; 6 bytes
+counter = $020a			; 2 bytes
+	
+E  = %10000000
+RW = %01000000
+RS = %00100000
+
+  .org $8000
+
+
+	
+reset:
+
+	ldx #$ff
+	txs
+	cli
+	
+	lda #%11111111 ; Set all pins on port B to output
+	sta DDRB
+	
+	lda #%11100000 ; Set top 3 pins on port A to output
+	sta DDRA
+	
+	lda #%00111000 ; Set 8-bit mode; 2-line display; 5x8 font
+	jsr send_inst
+	
+	lda #%00001110 ; Display on; cursor on; blink off
+	jsr send_inst
+	
+	lda #%00000001  ; Reset display
+	jsr send_inst
+
+	lda #%00000110 ; Increment and shift cursor; don't shift display
+	jsr send_inst
+
+
+loop:
+	lda#%00000010 		; home the cursor
+	jsr send_inst
+
+	lda #0
+	sta message
+	
+	;;  initialize value to the conversion number
+	sei
+	lda counter
+	sta value
+	lda counter + 1
+	sta value + 1
+	cli
+	
+divide:	
+	;; initialse remainder to zero
+	lda #0
+	sta mod10
+	sta mod10 + 1
+	clc
+	
+	ldx #16
+divloop:	
+
+	;; rotate the quotient
+	rol value
+	rol value + 1
+	rol mod10
+	rol mod10 + 1
+
+	;;  a,y = dividend = divisor
+	sec
+	lda mod10
+	sbc #10
+	tay 			; save low byte in Y
+	lda mod10 + 1
+	sbc #0
+	bcc ignore_result 	; branch if dividend < divosr
+
+	sty mod10
+	sta mod10 + 1
+	
+ignore_result:	
+	dex
+	bne divloop
+	rol value 		; shift in last bit of quotient
+	rol value + 1
+	
+	lda mod10
+	clc
+	adc #"0"
+	jsr push_char
+
+	;;  if value != 0 continue dividing
+	lda value
+	ora value + 1
+	bne divide 
+
+
+	  ldx #0
+print:
+	lda message,x
+	beq loop
+	jsr send_data 
+	inx
+	jmp print	
+
+	;; add character in A to start of message
+push_char:
+	pha 			; Push first char on stack
+	ldy #0
+char_loop:	
+	lda message,y 		; get char on string and put in X
+	tax
+	pla
+	sta message,y 		; pull char off stack and add it to the string
+	iny
+	txa
+	pha			; put char from string and put it back
+	bne char_loop
+
+	pla
+	sta message,y		; Pull null off stack stuff on end of string
+
+	rts
+
+	
+lcd_wait:
+	pha
+	lda #%00000000 ; PORT B to INPUT
+	sta DDRB
+lcd_busy:	
+	lda #RW
+	sta PORTA
+	lda #(RW | E)
+	sta PORTA
+	lda PORTB
+	and #%10000000
+	bne lcd_busy
+	
+	lda #RW
+	sta PORTA
+	lda #%11111111 ; PORT B to output
+	sta DDRB
+	pla
+	rts
+
+send_inst:	
+	jsr lcd_wait
+	sta PORTB
+	lda #0         ; Clear RS/RW/E bits	
+	sta PORTA
+	lda #E         ; Set E bit to send instruction
+	sta PORTA
+	lda #0         ; Clear RS/RW/E bits
+	sta PORTA
+	rts
+
+send_data:
+	jsr lcd_wait
+	sta PORTB
+	lda #RS         ; Set RS; Clear RW/E bits
+	sta PORTA
+	lda #(RS | E)   ; Set E bit to send instruction
+	sta PORTA
+	lda #RS         ; Clear E bits
+	sta PORTA
+	rts
+
+	
+nmi:
+irq:
+	inc counter
+	bne exit_irq
+	inc counter + 1	
+exit_irq:	
+	rti	
+
+.org $fffa
+.word nmi
+.word reset
+.word irq
+.word $0000