Stable Raster

Stable raster is the Commodore 64 technique for achieving cycle-exact synchronisation with the VIC-II’s raster beam. The jitter is not a bus-contention effect: it is that the CPU finishes whatever instruction it is executing before it takes the interrupt, and instructions are different lengths. That costs 0-7 cycles, and everything after it is fixed overhead — 7 cycles to enter the handler, 13 to push the registers, so the effect code starts somewhere in a 20-to-27-cycle window. 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 one line later
first_irq:
    lda #<second_irq
    sta $0314           ; Point CINV at second handler
    lda #>second_irq
    sta $0315
    inc $d012           ; Next IRQ one line down
    lda #$01
    sta $d019           ; Acknowledge
    tsx
    cli                 ; Re-enable IRQs (so second can fire)
    nop : nop : nop : nop : nop : nop : nop : nop
    ; ... long NOP slide ...
    ; The second IRQ will fire somewhere inside this slide,
    ; at one of two cycle positions (jitter is now 0 or 1).

; Second IRQ - stabilised
second_irq:
    ; Read $d012 to compensate for the remaining 0-1 cycle jitter
    lda $d012
    cmp $d012           ; If line just changed, branch to align
    beq @aligned
@aligned:
    ; Now cycle-exact — perform the effect
    sta $d020           ; Example: change border colour
    ; ... acknowledge and exit ...

The standard idiom is “first IRQ enables nested IRQ, NOP slide gives the second IRQ a known entry point, then a CMP+BEQ trick eliminates the last cycle of jitter.” This is the backbone of every cycle-exact C64 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 / $D016 bit-3 writes at cycle-precise raster positions
Raster bars	Colour changes per scanline
Split screens	Mode changes mid-frame (text/bitmap split, multicolour switch)
FLD (Flexible Line Distance)	Manipulate $D011 YSCROLL to space rendered lines vertically — produces the “stretchy screen” effect
FLI (Flexible Line Interpretation)	Per-line $D018 change combined with forced badlines — yields 8 colours per 8×8 cell
Tech-tech	Per-line $D016 XSCROLL change so text undulates in a sine wave

What breaks it

The reference routine states its own preconditions without softening them:

Other Interrupts, $d012 = #$ff, Sprites, Badline and Badline-1 = fuckup.

Sprites and badlines both steal cycles from the CPU, so a stable raster established outside them stops being stable inside them. The KERNAL is a further complication: “if you have the KERNAL banked in, you need to modify the IRQ-Vectors.”

It is region-specific

The NOP slide is tuned to the line length, so it does not survive the PAL/NTSC boundary. Codebase64’s routine carries three variants — PAL-63, NTSC-64, NTSC-65 — differing by a single loop count, and a note to “add one extra nop for 65 cycle NTSC machines”. The shortest known form, “the polling method, half variance technique”, makes the same switch in one byte.

This is the reason so much C64 demo code is PAL-only. It is not a preference; the timing does not carry.

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
• Demo Scene