Separate kernel code from test code

Eventually I want the kernel to be loaded from the SD card as well, but
it still needs to separate from user programs.

At some point there should be a folder just for the BIOS, which should
read from the boot block of the SD card and start executing, and thats
it.

sw/kernel/devices/interrupt.s

@@ -0,0 +1,58 @@
+; ---------------------------------------------------------------------------
+; interrupt.s
+; ---------------------------------------------------------------------------
+;
+; Interrupt handler.
+;
+; Checks for a BRK instruction and returns from all valid interrupts.
+
+.import   _handle_irq
+
+.export   _irq_int, _nmi_int
+.export   _irq_get_status, _irq_set_status
+
+.include "io.inc65"
+
+.segment  "CODE"
+
+.PC02                             ; Force 65C02 assembly mode
+
+; ---------------------------------------------------------------------------
+; Non-maskable interrupt (NMI) service routine
+
+_nmi_int:  RTI                    ; Return from all NMI interrupts
+
+; ---------------------------------------------------------------------------
+; Maskable interrupt (IRQ) service routine
+
+_irq_int:  PHX                    ; Save X register contents to stack
+           TSX                    ; Transfer stack pointer to X
+           PHA                    ; Save accumulator contents to stack
+           INX                    ; Increment X so it points to the status
+           INX                    ;   register value saved on the stack
+           LDA $100,X             ; Load status register contents
+           AND #$10               ; Isolate B status bit
+           BNE break              ; If B = 1, BRK detected
+
+; ---------------------------------------------------------------------------
+; IRQ detected, return
+
+irq:       PLA                    ; Restore accumulator contents
+           PLX                    ; Restore X register contents
+           jsr _handle_irq        ; Handle the IRQ
+           RTI                    ; Return from all IRQ interrupts
+
+; ---------------------------------------------------------------------------
+; BRK detected, stop
+
+break:     JMP break              ; If BRK is detected, something very bad
+                                  ;   has happened, so stop running
+
+_irq_get_status:
+           lda IRQ_STATUS
+           ldx #$00
+           rts
+
+_irq_set_status:
+           sta IRQ_STATUS
+           rts