Initial commit: C64 project skeleton with oscar64 submodule

2026-07-17 00:36:14 +02:00
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+# Raster Interrupts
+
+Source: https://www.c64-wiki.com/wiki/Raster_interrupt
+
+A raster interrupt is an interrupt trigger signal that the VIC-II can supply, if desired, to the CPU whenever the raster in the VIC's video signal reaches a specific line. With machine code programming, this mechanism can be exploited to perform many kinds of VIC "trickery" — having both text and high-res graphics on screen simultaneously, displaying more than eight hardware-supported sprites at once. This is heavily used on the C64/C128 for computer games and demos.
+
+The Atari 800, MSX, and Amstrad CPC also support raster interrupts. The 80-column mode on the C128/VDC cannot.
+
+## Setting up a raster interrupt
+
+By default, the system is set up to receive timer-based signals from CIA-1's Timer A. To use the VIC raster interrupt:
+
+```asm
+Init    SEI                     ; disable IRQ
+        LDA #%01111111
+        STA $DC0D               ; switch off interrupt signals from CIA-1
+
+        AND $D011               ; clear MSB of VIC raster
+        STA $D011
+
+        STA $DC0D               ; acknowledge pending CIA-1 IRQs
+        STA $DD0D               ; acknowledge pending CIA-2 NMIs
+
+        LDA #210                ; set raster line where interrupt shall occur
+        STA $D012
+
+        LDA #<Irq
+        STA $0314               ; set IRQ vector
+        LDA #>Irq
+        STA $0315
+
+        LDA #%00000001
+        STA $D01A               ; enable raster interrupt
+
+        CLI                     ; re-enable IRQ
+        RTS
+```
+
+Note: enabling raster interrupts from the VIC takes place *after* setting up everything else. The routine starts with `SEI` because if the interrupt is enabled before e.g. the vector is re-directed, an interrupt may occur while the vector is being altered, sending the CPU to a "random" address and crashing the system.
+
+## Single ISR example (wiggle border)
+
+```asm
+Irq     LDA #$07
+        STA $D020               ; border = yellow
+
+        LDX #$90                ; delay ~half a millisecond
+Pause:  DEX
+        BNE Pause
+
+        LDA #$00
+        STA $D020               ; border = black
+
+        ASL $D019               ; acknowledge raster IRQ
+        JMP $EA31               ; into KERNAL standard ISR (handles cursor blink, etc.)
+```
+
+The yellow stripe across the border is exactly 8 raster lines wide, set by the pause loop length.
+
+## Multiple raster ISRs (split-screen: hires top, text bottom)
+
+The "concatenated" pattern — two routines handling two different raster lines, each setting up the next:
+
+```asm
+Irq     LDA $D011
+        AND #%11011111
+        STA $D011               ; switch to text mode
+
+        LDA #<Irq2
+        STA $0314
+        LDA #>Irq2
+        STA $0315
+
+        LDA #$0
+        STA $D012               ; next IRQ at line 0
+
+        ASL $D019               ; acknowledge
+        JMP $EA31               ; into KERNAL ISR
+
+Irq2    LDA $D011
+        ORA #%00100000
+        STA $D011               ; switch to bitmap mode
+
+        LDA #<Irq
+        STA $0314
+        LDA #>Irq
+        STA $0315
+
+        LDA #210
+        STA $D012               ; next IRQ at line 210
+
+        ASL $D019
+        JMP $EA81               ; shorter ROM routine — just restore regs and RTI
+```
+
+Only one of the chained routines needs to jump to the full KERNAL ISR; the others can exit through $EA81 (the register-restore stub) for speed.