despatch.a

        ; despatch.a - interrupt despatcher for 6809

        include "via6522.i"
        include "globals.i"
		include "panic.i"

        ORG     $0000           ; This is a relocatable module.
		EXEC	init_despatcher

        ; 6522 VIA interrupt despatcher.
intdespatch:                    ; 6809 IRQ entry point.
		begin
			ldx     #g_unknownvec   ; Default to "unknown" vector.
			lda     ifr             ; Read interrupt flag register.
			sbpl    out             ; Branch if no interrupts pending.

			anda    ier             ; Mask with interrupt enable register.
			ora     #$80            ; Set last bit to ensure default vec.

loop:   	leax    -2,x            ; Loop through vectors
			asra                    ; testing each interrupt bit, LSB first.
			sbcc    loop            ; Loop unless the bit was set.

out:    	jmp     [,x]            ; Jump to the selected interrupt vector.
		end

default_isr:
		begin
			PANIC "Unhandled interrupt"
		end

        ; Initialisation routine to set up the interrupt despatcher.
init_despatcher:
		begin
									; ToDo: set up default ISRs for each vector.
			leax	default_isr,pcr        ; Copy the current IRQ vector
			stx		g_unknownvec    ; to the "unknown" vector.
			leax	intdespatch,pcr ; Put the address of the despatcher
			stx		g_irqvec        ; into the IRQ vector.
			rts                     ; Done, return to the loader.
		end