Overview
Every arcade machine in history has blinked its PRESS START — and the whole mechanism is one AND. A free-running frame counter already exists in any frame-locked game; a single bit of it toggles at a fixed beat (bit 5 flips every 32 frames ≈ 0.6s at 50/60 Hz). Test the bit: one way draws the prompt, the other clears its cells.
There is no blink state to manage, no timer to reset, nothing to get out of sync — the counter was already counting. Pick a higher bit for a slower wink, a lower bit for urgency.
The pattern generalises past blinking: any visual that alternates on a beat (a tremble, a pulse, a two-frame idle animation) is the same AND against the same counter, choosing between two renders.
Pseudocode
; somewhere in the per-frame loop (or already there)
frame_count += 1
; on the title screen
title_draw:
if frame_count AND %00100000: ; bit 5: 32 on, 32 off
clear the prompt cells
else:
draw "PRESS START"Implementation Notes
Where the writes happen matters more than the test. On the NES (Dash), nametable writes belong to vblank — so the main loop only increments blink_timer, and the NMI handler reads bit 5 and writes either the text tiles or eleven blanks through PPUDATA. Splitting clock from writes is the real lesson: the main loop owns time, the vblank handler owns VRAM.
On framebuffer machines (Amiga — Flock), the test and the writes can live together: move.w framecnt,d0 / and.w #32,d0 / beq draw / bsr clear. Flock draws its prompt through the same glyph routine the score uses, and clears with the same rectclear that built the farm.
Clear exactly what you drew. The clear path must blank precisely the prompt’s cells — clearing wider eats the background; clearing narrower leaves ghost pixels at the prompt’s edges.
Same clock, different effect: Flock’s frightened-sheep tremble is this pattern at bit 2 — a two-frame jitter selected by framecnt, no new state.
Trade-offs
| Aspect | Cost |
|---|---|
| CPU | One AND and a branch per frame, plus the draw/clear itself |
| Memory | Zero — the frame counter was already there |
| Complexity | One AND |
When to use: prompts, cursors, alarms, idle pulses — anything that should breathe on a beat.
When to avoid: when the player must not miss the message mid-blink — critical text stays solid.
Benefits
- No state — the clock is the state, and it can’t drift
- Tunable with one constant — the bit you test is the tempo
- Composable — different bits drive different effects from the same counter