Lesson 29 · FX07, FX15, FX18 - timer access

FX07, FX15, FX18 - timer access

Three opcodes connect programs to the timers - one reads the delay timer, two set the timers from a register. Today you wire all three.

The goal

Implement FX07 (read delay), FX15 (set delay), and FX18 (set sound).

Start here - the target

TO DO

Scenario: Programs read and set the timers

GivenV0 holds 0x09

When0xF015 runs (set delay = V0), then 0xF107 runs (V1 = delay)

Thenthe delay timer holds 0x09 and V1 becomes 0x09

And0xF018 sets the sound timer to V0 (0x09), leaving the delay timer unchanged

Background

The timers count down on their own, but a program needs to load and read them, and three FX opcodes do exactly that. FX15 sets the delay timer from VX; FX18 sets the sound timer from VX; FX07 copies the current delay timer back into VX. There is no opcode to read the sound timer - a program only ever writes it and lets it beep down - which is why the read side has just one instruction.

Together with the countdown from the last lesson, these give the classic timing loop: a program writes a value with FX15, then spins reading it with FX07 until it hits zero, producing a fixed delay regardless of CPU speed. All three sit in the 0xF family, told apart by their low byte, alongside the memory opcodes you build in the final chapter. The spec pins a round trip - set the delay to 0x09, read it straight back - so both directions are verified against the same value.

Make it work

// in the 0xF000 arm, sub-switch on the low byte:
case 0x07: v.V[x] = v.delay // read the delay timer into VX
case 0x15: v.delay = v.V[x] // set the delay timer from VX
case 0x18: v.sound = v.V[x] // set the sound timer from VX

CheckpointDONE

FX07, FX15, and FX18 read and set the delay and sound timers. Commit and stop here.