Minimise performance issues of None Maskable Interupt

wip-hugo/STARTUP.s

@@ -1,7 +1,7 @@
 .scope STARTUP
         ;;; -*- Mode: asm; indent-tabs-mode: t; tab-width: 8 -*-
         ;; Settings positions
-CR1 = $d011; ;Graphic settings
+        CR1 = $d011; Graphic settings
         VIC_bank_settings = $DD00 ; Vic position
         Screen_RAM_settings =$D018 ; Screen RAM position relative to VIC
 
@@ -27,7 +27,7 @@ Character_generator_ROM = $D000
 
         ;; Free upp memory
         ;; https://www.c64-wiki.com/wiki/Bank_Switching
-;;
+
         ;; Sets grafic mode [Standard bitmap mode]
         ;; https://www.c64-wiki.com/wiki/Standard_Bitmap_Mode
         LDA #%10111111 ; ECM = False
@@ -69,7 +69,9 @@ Mov_16 B_start, B_start + 1, #<Screen_RAM, #>Screen_RAM
         Mov_16 B_end, B_end + 1, #<Screen_RAM_end, #>Screen_RAM_end
         LDA #%11110000
         jsr memset
-SEI ;Disable interups (not all)
+
+        ;; Disable maskeble interups (not all)
+        SEI
         ;; Disable BASIC ROM mohahaha
         LDA #%11111110
         AND $0001
@@ -79,4 +81,29 @@ STA $0001
         LDA #%11111011
         AND $0001
         STA $0001
+
+        ;;Disable non maskable interupts
+        ;;https://codebase64.org/doku.php?id=base:nmi_lock_without_kernal
+        ;; write to $FFFA/$FFFB possible (and needed) if BASIC ROM is disabled
+
+        LDA #$00
+        STA $CCCC
+
+        LDA #<NMI_routine
+        STA $FFFA
+        LDA #>NMI_routine
+        STA $FFFB
+        LDA #$00  ;; stop Timer A
+        STA $DD0E
+        STA $DD04 ;; set Timer A to 0, after starting
+        STA $DD05 ;; NMI will occur immediately
+        LDA #$81
+        STA $DD0D ;; set Timer A as source for NMI
+        LDA #$01
+        STA $DD0E ;; start Timer A -> NMI
+          ;; from here on NMI is disabled
+        JMP NMI_routine_end
+NMI_routine:
+        RTI ;; exit interrupt not acknowledged
+NMI_routine_end:
 .endscope