Add interrupt handlers and redo vector locations

Most of these are taken from
https://cc65.github.io/doc/customizing.html, but modified to suit this
setup.

sw/boot.s

@@ -14,12 +14,6 @@
 
 .include  "zeropage.inc"
 
-.segment "VECTORS"
-
-.addr _init
-.addr _init
-.addr _init
-
 ; ---------------------------------------------------------------------------
 ; Place the startup code in a special segment
 
@@ -49,7 +43,7 @@ _init:    LDX     #$FF                 ; Initialize stack pointer to $01FF
 
 ; ---------------------------------------------------------------------------
 ; Call main()
-
+          cli
           JSR     _main
 
 ; ---------------------------------------------------------------------------

sw/interrupt.s

@@ -0,0 +1,46 @@
+; ---------------------------------------------------------------------------
+; interrupt.s
+; ---------------------------------------------------------------------------
+;
+; Interrupt handler.
+;
+; Checks for a BRK instruction and returns from all valid interrupts.
+
+.import   _handle_irq
+
+.export   _irq_int, _nmi_int
+
+.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

sw/irq.c

@@ -0,0 +1,7 @@
+
+// This is defined in main.c
+void puts(const char* s);
+
+void handle_irq() {
+    puts("Interrupt Detected!\n");
+}

sw/vectors.s

@@ -0,0 +1,14 @@
+; ---------------------------------------------------------------------------
+; vectors.s
+; ---------------------------------------------------------------------------
+;
+; Defines the interrupt vector table.
+
+.import    _init
+.import    _nmi_int, _irq_int
+
+.segment  "VECTORS"
+
+.addr      _nmi_int    ; NMI vector
+.addr      _init       ; Reset vector
+.addr      _irq_int    ; IRQ/BRK vector