ACIA interrupt handling

samples/echo_irq/echo_irq.asm

@@ -0,0 +1,48 @@
+;;
+;; Read input from the keyboard, and echo to console.
+;;
+
+
+IOBASE   = $8800
+IOSTATUS = IOBASE + 1
+IOCMD    = IOBASE + 2
+IOCTRL   = IOBASE + 3
+
+.segment "CODE"
+.org $C000
+
+START:  CLI
+        LDA #$09
+        STA IOCMD      ; Set command status
+        LDA #$1A
+        STA IOCTRL     ; 0 stop bits, 8 bit word, 2400 baud
+
+;;
+;; Infinite idle loop, waiting for interrupt.
+;;
+IDLE:   LDA #$FF
+        TAX
+IDLE1:  DEX
+        BNE IDLE1
+        JMP IDLE
+
+;;
+;; IRQ handler
+;;
+IRQ:    LDA IOBASE      ; Get the character in the ACIA.
+        PHA             ; Save accumulator
+ECHO1:  LDA IOSTATUS    ; Read the ACIA status
+        AND #$10        ; Is the tx register empty?
+        BEQ ECHO1       ; No, wait for it to empty
+        PLA             ; Otherwise, load saved accumulator,
+        STA IOBASE      ; write to output,
+        RTI             ; and return
+
+; system vectors
+
+.segment "VECTORS"
+.org    $FFFA
+
+.word   IRQ         ; NMI vector
+.word   START       ; RESET vector
+.word   IRQ         ; IRQ vector