The Machine Speaks Back

Back in Unit 8 the machine learned to hear you — reading the joystick through a register. This unit is the other half: the machine answering back. And on the C64, answering means the SID.

The SID (Sound Interface Device) is a proper synthesiser on a chip — three independent voices, each with its own pitch, waveform, and volume envelope. It's why C64 music still sounds like nothing else. You don't bang out the waveform yourself the way a lesser machine would; you set the chip's registers — the knobs on the synth — and it generates the sound on its own, holding the note while your code moves on.

A voice needs four things set: its pitch (a frequency), its envelope (how the note swells and fades — attack, decay, sustain, release), the master volume, and a waveform with the gate opened to start it. This unit plays one steady note on voice 1.

What you'll hear by the end

A steady, bright tone, held for as long as the machine runs. It's a sawtooth wave — the buzzy, reedy voice that carries a thousand C64 basslines. The waveform strip above is the real captured output.

Set the knobs, open the gate

; Meet the Machine - Unit 15: The Machine Speaks Back
; Assemble with: acme -f cbm -o sid.prg sid.asm
;
; The SID is the C64's sound chip - a three-voice synthesiser. You set a voice's
; pitch, its envelope, and the volume, then pick a waveform and open the "gate".
; The chip holds the note on its own while the CPU does nothing.

*= $0801
!byte $0c,$08,$0a,$00,$9e,$32,$30,$36,$31,$00,$00,$00   ; 10 SYS 2061

*= $080d

        ; a background colour, so the screen shows it's running
        lda #$00
        sta $d020           ; border black
        lda #$06
        sta $d021           ; background blue

        ; clear the start-up text away to a blank screen
        ldx #0
clear   lda #$20            ; space
        sta $0400,x
        sta $0500,x
        sta $0600,x
        sta $0700,x
        inx
        bne clear

        ; ----------------------------------------------- YOUR CODE START
        ; --- set up SID voice 1 ---
        lda #$0f
        sta $d418           ; master volume to maximum (0-15)

        lda #$00
        sta $d400           ; frequency, low byte
        lda #$11
        sta $d401           ; frequency, high byte  -> the pitch

        lda #$00
        sta $d405           ; attack 0, decay 0
        lda #$f0
        sta $d406           ; sustain 15, release 0  -> the note holds at full

        lda #$21
        sta $d404           ; sawtooth waveform + gate ON  -> the note starts
        ; ------------------------------------------------- YOUR CODE END

loop    jmp loop            ; the CPU idles; the SID plays on by itself

The sound is one block of stores to the SID's registers, which live from $D400:

lda #$0f
sta $d418           ; master volume to maximum (0-15)

lda #$00
sta $d400           ; frequency, low byte
lda #$11
sta $d401           ; frequency, high byte  -> the pitch

lda #$00
sta $d405           ; attack 0, decay 0
lda #$f0
sta $d406           ; sustain 15, release 0  -> the note holds at full

lda #$21
sta $d404           ; sawtooth waveform + gate ON  -> the note starts

Every line is a sta to a SID register — you're turning knobs on a synthesiser. $D418 is the master volume. $D400/$D401 set the pitch as a 16-bit frequency, low byte then high (the two-byte numbers from Unit 14). $D405/$D406 set the envelope: with sustain at maximum and release at zero, the note holds at full level for as long as it's gated.

The last store does two things at once. $D404 is voice 1's control register; $21 selects the sawtooth waveform (bit 5) and opens the gate (bit 0). Opening the gate is what starts the note — and because nothing ever closes it, the note rings on while the CPU sits in loop. You set the chip going and walked away; the SID does the rest.

Assemble and run

acme -f cbm -o sid.prg sid.asm

A steady sawtooth tone; a blank blue screen.

Try this: change the pitch

$D400/$D401 are the frequency. Raise the high byte — lda #$22 instead of #$11 — and the note jumps roughly an octave; lower it and the note drops. The number is the pitch; the SID turns it into a tone.

Try this: a different voice

Change $D404 from $21 to $11 (triangle + gate) for a softer, flute-like note, or $41 (pulse + gate) for a hollow square — though pulse also wants a width set in $D402/$D403. Same chip, same four steps; a different waveform bit gives a different timbre.

If it doesn't work

• Silence. Volume at $D418 is zero, or the gate never opened — the control write to $D404 must have bit 0 set ($21, not $20).
• A click, then nothing. The envelope released. With release in the low nibble of $D406 set above zero, the note fades after the gate; keep sustain high and release at zero for a held tone.
• A tone that never stops even when you mean it to. That's the gate staying open — clearing bit 0 of $D404 (write $20) closes the gate and starts the release. Sound is something you turn off as well as on.

What you've learnt

The SID is the C64's three-voice synthesiser. You set a voice's pitch ($D400/$D401), its envelope ($D405/$D406), the master volume ($D418), then write a waveform plus the gate to $D404 to start the note. The chip holds it on its own while the CPU is free. You turn the knobs; the synth plays.

What's next

That's the last of what the machine can do — you've now driven its screen, its colour, its input, and its sound. One unit remains, and it isn't about a new instruction. Next — The Machine Trusts You — what it means to program a machine that does exactly what you wrote, even when you're wrong.