MOS Sound Interface

The MOS-level sound API: OSWORD &07 (SOUND), OSWORD &08 (ENVELOPE), suppression OSBYTEs, BELL. For direct chip writes, see sn76489.

OSWORD &07 — SOUND

LDX #params MOD 256
LDY #params DIV 256
LDA #&07
JSR &FFF1

Parameter block (8 bytes, all 16-bit little-endian):

Offset	Field
0-1	Channel
2-3	Amplitude
4-5	Pitch
6-7	Duration

Channel encoding

The 16-bit channel value carries more than just “which channel”:

• Low nibble (bits 0-3): channel number 0-3 (0 = noise, 1-3 = tones).
• High bits: flush / sync / queue control:
  • &00xx — queue normally.
  • &FFxx, &FExx, &FDxx, &FCxx — speech-channel codes (when speech hardware present, channel goes to the speech chip via OSBYTE &9F).
  • Bits in the top byte select “flush before queueing”, channel sync, and (on speech) phrase selection.

NAUG §21.1.1 p372 documents the parameter shape but defers to the BASIC User Guide for the full encoding semantics. Capture more if needed.

Amplitude

&FF00-&FF0F = BASIC amplitudes -15..-0 = chip volumes 15..0. Or pass an envelope number (1-16) in the amplitude field to attach an ENVELOPE to this sound.

Pitch

Frequency, 0-255. BBC pitch unit ≈ 1/4 semitone. Pitch 0 = ~7 Hz, pitch 255 ≈ Hz to come. The MOS converts pitch to the 10-bit divider N for the chip (sn76489).

Duration

Length in 20 ms units (1/50th sec). -1 = play indefinitely.

OSWORD &08 — ENVELOPE

LDX #params MOD 256
LDY #params DIV 256
LDA #&08
JSR &FFF1

14-byte parameter block — same 14 parameters as BASIC’s ENVELOPE command (envelope number, step length, pitch slopes, amplitude slopes, etc.). NAUG defers the full breakdown to the User Guide.

After definition, the envelope is referenced by number in the amplitude field of OSWORD &07.

Sound suppression

OSBYTE	Function	Encoding
&D2 (210)	Sound on/off	0 = enabled (default); ≠0 = disabled. Standard <NEW>=(<OLD> AND Y) EOR X.
&D1 (209)	Speech on/off	&50 (SPEAK) = enabled, &20 (NOP) = disabled.
&EB (235)	Speech presence	Read-only effectively; 0 = absent, &FF = present.

CTRL-G BELL parameters

The &07 ASCII bell. Configured via:

OSBYTE	Field	Default
&D3 (211)	Channel	3
&D4 (212)	Amplitude / envelope	144 ((-13 - 1) * 8 + 256 = 144, envelope 0 = amplitude -13)
&D5 (213)	Frequency / pitch	100
&D6 (214)	Duration	6

NAUG §21.1.7 p374 documents the amplitude encoding: ((amp_or_env - 1) * 8) mod 256, with low 3 bits = H/S parameters of the channel.

Sound buffer ↔ chip pipeline

1. OSWORD &07 parses parameters, enqueues into the appropriate channel buffer (4-7, buffers).
2. 100 Hz IRQ (System VIA T1, interrupts) services buffers:
   • Pulls next note.
   • Applies envelope (pitch + amplitude steps).
   • Writes to the SN76489 via the slow-bus dance (sn76489).
3. Note plays for its duration; envelope continues stepping per 100 Hz tick.

Latency from OSWORD &07 to first audible output: up to 10 ms (next 100 Hz tick). For lower latency, write the chip directly (sn76489).

When to bypass MOS

Use case	Path
BASIC-style music, simple SFX	OSWORD &07 + OSWORD &08
Game sound with envelopes	Mostly OSWORD &07; reserve channel 0 for direct hits
Sample playback (PCM-ish)	Direct sn76489, timer-driven
Custom music engine (tracker / SID-style)	Mostly direct
Sub-frame-timed FX	Direct

The 100 Hz envelope step is the speed ceiling for MOS-driven sound. Anything faster needs direct chip writes paced by a VIA timer.

See also

• sn76489 — Chip-level reference.
• system-via — Slow-bus protocol (sound /WE on latch line 0).
• buffers — Channel buffers 4-7.
• interrupts — 100 Hz T1 IRQ that services the buffers.
• via-timers — Timer-driven direct sound writes.

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