| name | c64-sid |
| description | The MOS 6581 SID sound chip and making sound and music on the Commodore 64 — the register map at $D400-$D41C, the three voices, the four waveforms (triangle, sawtooth, pulse, noise), the ADSR envelope generator, pulse width, the multimode filter, master volume, ring modulation and oscillator sync, and the frequency/note tables. Use this skill WHEN asked to "play a note/sound on the C64", "program the SID", "what does $D418 do", "set up an ADSR envelope", "make a sound effect", "why is my SID silent", or any POKE/STA into $D4xx. Pairs with c64-game-ports (SID reads the paddles), c64-memory-map, c64-vic-ii and c64-cia (raster/timer-driven music players). |
SID 6581 — sound and music
The SID sits at $D400 (54272). It has 3 independent voices, each with a tone
oscillator (4 selectable waveforms), an ADSR envelope, and a route into one shared
multimode analog filter, plus a master volume. Most registers are write-only —
you cannot read back what you POKEd; only $D419-$D41C are readable. Getting sound
out is mostly: set volume, set an envelope, set a frequency, then gate the
voice on.
In BASIC people use a base variable S = 54272 and POKE S+offset. In assembly
you STA $D4xx. Both are shown below.
Register map ($D400 + offset)
Voice block repeats at base +0 (voice 1), +7 (voice 2 → $D407),
+14 (voice 3 → $D40E):
| Off | Reg | Name | Contents |
|---|
| +0 | $D400 | FREQ LO | oscillator frequency, low byte |
| +1 | $D401 | FREQ HI | oscillator frequency, high byte |
| +2 | $D402 | PW LO | pulse width, low byte |
| +3 | $D403 | PW HI | pulse width, high nibble (12-bit total) |
| +4 | $D404 | CONTROL | `NOISE PULSE SAW TRI |
| +5 | $D405 | ATK/DCY | attack (hi nibble) / decay (lo nibble) |
| +6 | $D406 | SUS/REL | sustain (hi nibble) / release (lo nibble) |
Filter & global (shared by all voices):
| Reg | Name | Contents |
|---|
| $D415 | FC LO | filter cutoff, low 3 bits |
| $D416 | FC HI | filter cutoff, high 8 bits (11-bit cutoff) |
| $D417 | RES/FILT | `RES3-0 |
| $D418 | MODE/VOL | `3OFF HP BP LP |
| $D419 | POTX | paddle X (read-only) — see c64-game-ports |
| $D41A | POTY | paddle Y (read-only) |
| $D41B | OSC3 | voice-3 oscillator output / random (read-only) |
| $D41C | ENV3 | voice-3 envelope output (read-only) |
The control register ($D404 / +4) — bit by bit
| Bit | Val | Name | Effect |
|---|
| 0 | 1 | GATE | 1 = start ATTACK/DECAY/SUSTAIN; 0 = begin RELEASE |
| 1 | 2 | SYNC | hard-sync this oscillator to the previous voice's oscillator |
| 2 | 4 | RING MOD | ring-modulate this voice's triangle with the previous voice's osc |
| 3 | 8 | TEST | reset & lock the oscillator (also used to silence noise lock-up) |
| 4 | 16 | TRIANGLE | select triangle waveform (soft, few harmonics) |
| 5 | 32 | SAWTOOTH | select sawtooth (rich, buzzy) |
| 6 | 64 | PULSE | select pulse — duty cycle set by PW LO/HI |
| 7 | 128 | NOISE | select noise (explosions, wind, hi-hats) |
Pick one waveform bit + GATE. E.g. sawtooth on = 32+1 = 33; pulse on = 65.
ADSR envelope
$D405 = attack:decay (one nibble each), $D406 = sustain:release. Attack/decay/
release are rates (0–15), sustain is a level (0 = silent … 15 = peak). The
rate→time table (from the chip spec):
| Value | Attack | Decay/Release |
|---|
| 0 | 2 ms | 6 ms |
| 2 | 16 ms | 48 ms |
| 5 | 56 ms | 168 ms |
| 9 | 250 ms | 750 ms |
| 11 | 800 ms | 2.4 s |
| 15 | 8 s | 24 s |
(Full 16-row table in references/sid-chip-spec.md, Table 2.) The envelope only
advances while GATE is 1; clearing GATE jumps to RELEASE. That gate edge is the
single most common reason a SID program is silent or stuck.
Frequency
Output frequency: Fout = (Fn × Fclk) / 16777216, where Fn is the 16-bit value
in FREQ HI:LO and Fclk ≈ 1,022,727 Hz (NTSC) / 985,248 Hz (PAL). You normally don't
compute it — use the note table: references/music-note-values.md lists Note#,
note name, decimal Fn, and the HI/LO bytes to POKE. (PAL pitch is ~4% lower for the
same Fn; retune if it matters.)
Play one note
BASIC:
10 S=54272
20 FOR L=S TO S+24: POKE L,0: NEXT : REM clear all SID registers
30 POKE S+24,15 : REM master volume = 15
40 POKE S+5,9 : POKE S+6,0 : REM attack=0 decay=9, sustain=0 release=0
50 POKE S+1,17: POKE S,37 : REM freq HI=17 LO=37 (a mid note)
60 POKE S+4,33 : REM sawtooth (32) + gate on (1)
70 FOR T=1 TO 300: NEXT
80 POKE S+4,32 : REM gate off -> release
Assembly:
LDA #$0F : STA $D418 ; master volume
LDA #$09 : STA $D405 ; attack 0 / decay 9
LDA #$00 : STA $D406 ; sustain 0 / release 0
LDA #$25 : STA $D400 ; freq lo
LDA #$11 : STA $D401 ; freq hi
LDA #$21 : STA $D404 ; sawtooth + GATE on (%00100001)
; ... delay ...
LDA #$20 : STA $D404 ; GATE off -> release
Filter, ring mod, sync
- Filter: set cutoff in
$D415/$D416, resonance + per-voice routing bits in
$D417, then choose mode in $D418 (bit4 LP, bit5 BP, bit6 HP — combinable; bit7
3OFF disconnects voice 3 from the output, handy when using voice 3 only as a
modulation source via OSC3/ENV3). A voice is only filtered if its FILT bit in
$D417 is set.
- Ring modulation: set RING MOD (bit2) and triangle on voice n; it is
modulated by voice n−1's oscillator (voice 1 ← voice 3). Metallic, bell tones.
- Sync: set SYNC (bit1) on voice n to hard-sync it to voice n−1's
oscillator. Both effects need the modulating voice's frequency set even if it's
silent.
Gotchas
- Silent output? Check: master volume
$D418 low nibble > 0; a waveform bit set;
GATE toggled on; a non-zero envelope (all-zero ADSR + sustain 0 can be inaudible).
- Registers are write-only — keep your own shadow copy if you need to read state.
- Pulse width only matters with the PULSE waveform selected.
- Don't combine multiple waveform bits casually; mixing noise with another
waveform can lock the noise generator silent until a TEST-bit or RES reset.
- Volume is global — there is no per-voice volume; balance with envelopes/filter.
References
references/sid-chip-spec.md — PRG Appendix O: the authoritative register
map, every bit's meaning, the full Table 2 envelope rates, filter math, and the
SID pinout. Read for exact semantics and edge cases.references/sound-programming.md — PRG Ch4: the programmer's walk-through of
volume, multiple voices, waveforms, the envelope, filtering, and advanced
sync/ring-mod technique with worked BASIC.references/sound-tutorial.md — User's Guide Ch7: gentle intro and sample
sound-effect programs.references/music-note-values.md — PRG Appendix E: Note# → note → Fn → HI/LO
bytes. The lookup table for playing tunes.references/sound-control-settings.md — User's Guide Appendix P: ready-made
register-setting tables for instrument-like sounds and effects.
Paddle reading via $D419/$D41A is documented in c64-game-ports. For music
players driven off a raster or CIA-timer interrupt, see c64-vic-ii and
c64-cia. SID register addresses and banking live in c64-memory-map.
