Game 3 Unit 1 of 6 1 hr learning time

The Flash

Reflex begins with a signal. Hold the screen black, wait a beat, then flash it white — the moment the player reacts to. One register does it, and a timing loop sets the wait.

17% of Reflex

Every reaction game needs a signal — the thing the player is waiting for. Reflex uses the boldest one the C64 has: the whole screen turning from black to white in a single instant. So start there. Hold the screen black, wait a beat, then flash it white.

10 PRINT CHR$(147)
20 POKE 53281,0
30 PRINT "WATCH THE SCREEN..."
40 FOR T=1 TO 800:NEXT T
50 POKE 53281,1
A C64 screen flashed fully white, the words WATCH THE SCREEN at the top.
The signal: black for a beat, then the whole screen white. This is the moment the player reacts to — and it's one POKE away.

Line 20 sets the screen black and line 50 sets it white, both with POKE 53281 — the register that holds the background colour, the colour filling the whole screen behind the text. Write a number to it and the screen changes that instant: 0 is black, 1 is white. You met this register in the primer; here it does real work.

Between them, line 40 is the wait: FOR T=1 TO 800:NEXT T. The loop counts to 800 and does nothing else — no drawing, no sound, just time passing. That's a timing loop, and it's how BASIC waits when it has nothing else to do. Eight hundred is about a second on the C64; change it and the beat moves.

Try this

  • Stretch the beat. Change 800 to 2000 and the screen sits black far longer before it flashes. Change it to 200 and the flash comes almost at once.
  • A different signal. Flash the screen red (POKE 53281,2) or yellow (POKE 53281,7) instead of white. Which one grabs your eye fastest?

What's next

The flash works — but it comes at the same moment every time, so you can learn to expect it. A reaction test you can predict is no test at all. In Unit 2, RND makes the wait a different length every run.