Input & Timers

CHIP-8 keypad: 16 keys laid out:

1 2 3 C
4 5 6 D
7 8 9 E
A 0 B F

Modern emulators map to keyboard, e.g.:

1 2 3 4    ->  1 2 3 C
Q W E R    ->  4 5 6 D
A S D F    ->  7 8 9 E
Z X C V    ->  A 0 B F

Input instructions:

• EX9E: skip next if key VX is pressed.
• EXA1: skip next if key VX is NOT pressed.
• FX0A: wait for key press, store in VX. Blocks until any key pressed.

Implementation:

Game loop calls update_keys() from real input, then runs CPU cycle.

Timers:

• Delay timer (DT): decremented at 60 Hz down to 0. No effect at 0.
• Sound timer (ST): decremented at 60 Hz. While > 0, output a beep (one tone).

Used for:

• DT: pacing — game waits N ticks before advancing.
• ST: sound effects (one note, fixed pitch).

LD V0, 60     ; 1 second
LD DT, V0     ; FX15
loop:
    LD V1, DT  ; FX07
    SE V1, 0
    JP loop

This loops until DT hits 0 = 1 second wait.

Decrement happens regardless of CPU state. In emulator: real-time clock or per-cycle counter.

Called 60 times per second, separate from CPU cycles.

Sound:

• Original CHIP-8: single-tone buzzer.
• Modern emulators: play 440 Hz square wave or generic beep when ST > 0.

Random:

• CXKK: VX = (random byte) AND KK.
• Used for non-deterministic game elements.
• Use real RNG; deterministic seed for testing.