Stable Raster

Overview

Stable raster is the Commodore 64 technique for achieving cycle-exact synchronisation with the VIC-II’s raster beam. Because the CPU and VIC-II share the bus, interrupt timing has inherent jitter (0-7 cycles). The stable raster technique eliminates this jitter, enabling perfectly timed mid-screen effects.

Fast Facts

Aspect	Detail
Problem	IRQ timing varies by 0-7 cycles
Solution	Double-IRQ with self-modifying code
Precision	Cycle-exact (0 jitter)
Used for	Border removal, split screens, colour bars

The Jitter Problem

When a raster interrupt fires, the CPU might be mid-instruction:

Instruction Length	Jitter
2 cycles	0-2 cycle variation
3 cycles	0-3 cycle variation
7 cycles	0-7 cycle variation

This jitter causes visible glitches in timing-sensitive effects.

The Double-IRQ Solution

The technique uses two interrupts:

IRQ	Purpose
First IRQ	Sets up second IRQ one line later
Second IRQ	Triggers inside a long NOP slide

Step by Step

1. First IRQ fires with unknown jitter
2. Set second IRQ for next raster line
3. Execute NOP slide (many 2-cycle NOPs)
4. Second IRQ fires inside NOP slide
5. Jitter is now only 0-1 cycles
6. Measure exact cycle position
7. Compensate with calculated delay

Code Structure

; First IRQ - unstable, sets up second
first_irq:
    lda #<second_irq
    sta $0314
    inc $d012        ; Next line
    ; ... acknowledge IRQ
    rti

; Second IRQ - stabilised
second_irq:
    ; NOP slide for consistent entry point
    nop : nop : nop : nop : nop : nop : nop
    ; Now cycle-exact
    ; ... do stable raster effect

Why It Works

Mechanism	Effect
NOP slide	All instructions same length (2 cycles)
Maximum jitter = 1	Only 0 or 1 cycle variation
Cycle counting	Determine exact position
Branch compensation	Conditional delay to align

Applications

Effect	Requirement
Border removal	Exact $D011 writes
Raster bars	Colour changes per line
Split screens	Mode changes mid-frame
FLD effects	Flexible Line Distance
Tech-tech	Multi-colour text

Historical Development

Era	Advancement
1986	Basic double-IRQ discovered
1987	Refined techniques
1990s	Demo scene mastery
Today	Still essential for C64 effects

Performance Cost

Factor	Impact
CPU time	Two IRQs per stable point
Code space	NOP slides, handler code
Complexity	Requires deep understanding

Modern Relevance

Understanding stable raster teaches:

• Interrupt handling fundamentals
• Cycle-exact timing
• Hardware synchronisation
• Why modern GPUs use VSync

See Also

• VIC-II
• Raster Tricks 101
• Demo Scene