c64-sid

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.