add interrupt example

interrupt.h

@@ -0,0 +1,29 @@
+// reserve locations where the 6502 jumps on IRQ
+#pragma zp_reserve(0,1,2)
+
+// reads the status register on the TMS9918 thus acknowledging the interrupt
+inline void acknowledge_interrupt() {
+   asm { lda VDP_REG };
+}
+
+volatile word tick_counts;
+
+__interrupt(hardware_all) void interrupt_handler() {
+   tick_counts++;
+
+   if(tick_counts == 60) {
+      tick_counts = 0;
+      woz_putc('T');   // print a T every second
+   }
+
+   // acknowledge interrupt by reading the status register
+   acknowledge_interrupt();
+}
+
+void install_interrupt() {
+   asm { sei };                                   // disable 6502 interrupts
+   *((byte *)0x0000) = 0x4C;                      // $4C = JUMP opcode
+   *((word *)0x0001) = (word) &interrupt_handler; // JUMP interrupt_handler
+   asm { cli };                                   // re-enable 6502 interrupts
+   write_reg(1, 0xc0 | 32);                       // turn on IE bit (interrupt enable) on the TMS9918
+}

test.c

@@ -82,27 +82,6 @@ IRQ on the apple1 goes to 0
 - DOKE 1,address routine
 */
 
-/*
-word tick_counts;
-
-__interrupt(hardware_all) void interrupt_handler() {
-   tick_counts++;
-}
-
-// reserve locations for the IRQ vector
-#pragma zp_reserve(0,1,2)
-
-//__address(0x0001) volatile word KERNEL_IRQ;
-
-void install_interrupt() {
-   asm { sei }
-   POKE(0,0x4C);
-   //KERNEL_IRQ = (word) &interrupt_handler;
-   // *((word *)0x0001) = (word) &interrupt_handler;
-   asm { cli }
-}
-*/
-
 word screen1_cursor;
 
 #include "laser500_font.ascii.c"
@@ -631,14 +610,24 @@ void vti_ellipse_rect(byte _x0, byte _y0, byte _x1, byte _y1)
 }
 */
 
+#include "interrupt.h"
+
+void prova_interrupt() {
+   // la seguente linea è un workaround temporaneo a causa di un bug di KickC
+   // fa uso della variabile tick_counts in modo che non venga ottimizzata
+   *((word *)0xFFFE) = tick_counts;
+
+   install_interrupt();
+}
+
 void main() {
-   //install_interrupt();
 
    byte key = '1';
    for(;;) {
            if(key == '1')  prova_screen1();
       else if(key == '2')  prova_screen2();
       else if(key == '3')  prova_screen3();
+      else if(key == '4')  prova_interrupt();
       else if(key == '0')  break;
       else woz_putc(key);