Implement interrupt status register

Upon an interrupt, you can read from the interrupt status register to
see what caused the interrupt.

sw/interrupt.h

@@ -0,0 +1,13 @@
+#ifndef _INTERRUPT_H
+#define _INTERRUPT_H
+
+#include <stdint.h>
+
+#define BUTTON  (1 << 0)
+
+void irq_int();
+void nmi_int();
+
+uint8_t irq_get_status();
+
+#endif

sw/interrupt.s

@@ -9,6 +9,9 @@
 .import   _handle_irq
 
 .export   _irq_int, _nmi_int
+.export   _irq_get_status
+
+.include "io.inc65"
 
 .segment  "CODE"
 
@@ -43,4 +46,9 @@ irq:       PLA                    ; Restore accumulator contents
 ; BRK detected, stop
 
 break:     JMP break              ; If BRK is detected, something very bad
-                                  ;   has happened, so stop running
+                                  ;   has happened, so stop running
+
+_irq_get_status:
+           lda IRQ_STATUS
+           ldx #$00
+           rts

sw/io.inc65

@@ -4,3 +4,5 @@ UART        = $7ff4
 UART_TXB    = UART
 UART_RXB    = UART
 UART_STATUS = UART + 1
+
+IRQ_STATUS  = $7fff

sw/irq.c

@@ -1,7 +1,19 @@
 
+#include <stdint.h>
+
+#include "interrupt.h"
+
 // This is defined in main.c
 void puts(const char* s);
 
 void handle_irq() {
+    uint8_t status;
+
     puts("Interrupt Detected!\n");
+
+    status = irq_get_status();
+
+    if (status & BUTTON) {
+        puts("Button Interrupt!\n");
+    }
 }

sw/tests/test_main.c

@@ -2,6 +2,7 @@
 
 #include "sevenseg.h"
 #include "uart.h"
+#include "interrupt.h"
 
 int main(void)
 {
@@ -75,5 +76,14 @@ int main(void)
     printf("Done!\n\n");
 
 
+    printf("Testing irq_get_status...\n");
+    *(uint8_t*)0x7fff = 0xa5;
+    if (irq_get_status() != 0xa5) {
+        printf("Incorrect value!\n", i);
+        retval++;
+    }
+    printf("Done!\n\n");
+
+
     return retval != 0;
 }