Ben Eater 6502 Kit — Day 22

The interrupt requests are meant to be shared between several advanced devices. The breadboard 65C02 computer has only one of those: The 65C22 VIA (versatile interface adapter). First we need to attach a button to the VIA and connect to VIAs IRQ-Out with the CPUs IRQ-In. I'll add a 2nd button and keep the NMI button for now:

2nd interrupt button.

Next we need a new IRQ handler. I'm going to write the the program in expandable way that multiple IRQs can be handled. Resulting in another over engineered software.

;;
;   Increment Counter
;
.proc     Inc_Counter
    INC  Counter   ; 16 bit increment
    BNE  Exit
    INC  Counter + 1
Exit:     RTS
.endproc

;;
;   NMI (non-maskable interrupt) handler
;
.proc     Do_NMI
    JSR  Inc_Counter  ; just increment the counter
    RTI
.endproc

;;
;   Handle interrupts from VIA
;
.proc     Do_VIA_IRQ
    PHA
    LDA  #%00000010  ; Check CA1 interrupt.
    BIT  VIA::IFR
    BZE  Next   ; Interrupt wasn't CA1, check next
    JSR  Inc_Counter  ; increment the counter
    BIT  VIA::ORA  ; Clear CA1 interrupt

Next:     PLA     ; No more interrupt sources to check.
    RTS
.endproc

;;
;   IRQ (interrupt request) hander
;
.proc     Do_IRQ
    BIT  VIA::IFR
    BPL  Next   ; Interrupt wasn't from VIA, check next
    JSR  Do_VIA_IRQ
Next:     RTI     ; No more interrupt sources to check.
.endproc

I first check if the IRQ originates from the VIA, using the BIT operation, and if so call a subroutine for VIA IRQs. I use a subroutine as branch commands can only jump 127 addresses forward and as the program get's more complex I would need to change to JMPs instead.

The VIA itself also has several interrupt sources to next I check if the IRQ came from them CA1 line. The BIT operation can check bit 6 and 7 without the accumulator for bit 1 I need to load the accumulator with the bit I wish to test.

And here the IRQ handler in stunning action.

All wired up and running