Introduce NMIs

homebrew-tutorial/main.asm

@@ -7,6 +7,10 @@
 
 .include "header.asm"
 
+.segment "ZEROPAGE"
+
+nmi_count:  .res 2
+
 .segment "CODE"
 
 VRAM_CHARS = $0000
@@ -93,10 +97,18 @@ start:
     lda #$0f
     sta INIDISP
 
-    lda #$01
+    lda #%10000001
     sta NMITIMEN
 
 mainloop:
+
+    lda nmi_count
+    nmi_check:
+        wai
+        cmp nmi_count
+        beq nmi_check
+
+
     lda JOY1L
     bit #JOYL_L
     beq down_not_pressed
@@ -117,6 +129,7 @@ busywait:
 
 nmi:
     bit RDNMI
+    inc nmi_count
 _rti:
     rti