amiga-bootcamp/01_hardware/ocs_a500/paula_audio.md

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Paula — Audio DMA

Overview

Paula (MOS 8364) provides four independent audio DMA channels (0–3), corresponding to the four hardware voice outputs:

Channel	Stereo Output
0	Left
1	Right
2	Right
3	Left

Each channel has its own DMA pointer, length, period (pitch), and volume registers. Paula fetches sample data from Chip RAM automatically at the rate set by the period register.

Audio Registers Per Channel

Channel n (n = 0–3) registers at $DFF0A0 + n×$10:

Offset	Name	Description
+$00/$01	AUDnLCH/AUDnLCL	Sample pointer high/low (Chip RAM address)
+$04	AUDnLEN	Sample length in words (not bytes)
+$06	AUDnPER	Period — clock divider for playback rate
+$08	AUDnVOL	Volume: 0–64 (0 = silent, 64 = full)
+$0A	AUDnDAT	Current sample word (direct, for non-DMA mode)

Sample Playback Rate

Paula derives playback rate from the system clock divided by the period register:

Sample rate (Hz) = System clock / Period

System	Clock	Period for 8363 Hz	Period for 22050 Hz
PAL	3,546,895 Hz	428	~161
NTSC	3,579,545 Hz	428	~162

8363 Hz is the standard MOD tracker reference — period 428 on PAL plays middle-A at exactly 8363 Hz.

Period range: 124–65535 (minimum period = maximum frequency ≈ 28 kHz PAL).

Starting Audio DMA

; Load channel 0 with sample at SampleData, length 256 words
move.l  #SampleData, d0
move.w  d0, AUD0LCL+custom
swap    d0
move.w  d0, AUD0LCH+custom
move.w  #256, AUD0LEN+custom    ; 256 words = 512 bytes
move.w  #428, AUD0PER+custom    ; period 428 = 8287 Hz PAL
move.w  #64,  AUD0VOL+custom    ; full volume

; Enable audio DMA for channel 0
move.w  #$8201, DMACON+custom   ; SET + MASTER + AUD0EN

; Enable audio interrupt (optional)
move.w  #$A080, INTENA+custom   ; SET + INTEN + AUD0

ADKCON — Audio/Disk Control

Bit	Name	Description
15	SET/CLR	Write: 1=set, 0=clear bits
11	PRECOMP1	Disk precompensation
10	PRECOMP0	Disk precompensation
9	MFMPREC	MFM precompensation
7	MSBSYNC	Audio sync from MSB
6	WORDSYNC	Disk word sync enable
4	ATPER	Channel 3 period from channel 2 data
3	ATVOL	Channel 3 volume from channel 2 data
2	ATPER2	Channel 1 period from channel 0 data
1	ATVOL2	Channel 1 volume from channel 0 data
0	(unused)

ATPER/ATVOL bits enable audio modulation — channel N's period/volume is modulated by the sample data of channel N-1. Used for AM synthesis effects (e.g., in many MOD players for 8-channel soft mixing).

Interrupt Handling

Audio channels raise interrupts when their DMA pointer wraps to the beginning of the next buffer:

INTENA/INTREQ bits:
AUD0 = bit 7  (IPL 4)
AUD1 = bit 8  (IPL 4)
AUD2 = bit 9  (IPL 4)
AUD3 = bit 10 (IPL 4)

The interrupt fires when the channel has consumed its buffer and reloaded the pointer from the DMA registers. At this point, software can update AUDnLCH/L and AUDnLEN with the next buffer.

Double-Buffering Pattern

/* In interrupt handler (IPL 4 server for AUD0): */
void audio_interrupt(void)
{
    /* Swap buffers */
    UWORD *next = (active_buf == buf_a) ? buf_b : buf_a;
    active_buf = next;

    /* Load next buffer address */
    custom.aud[0].ac_ptr = (UWORD *)next;
    custom.aud[0].ac_len = BUF_WORDS;
}

Direct (Non-DMA) Audio

For simple sound effects without DMA overhead, write directly to AUDnDAT:

move.w  #$0080, AUD0VOL+custom   ; volume 64 (approx)
move.w  #$7FFF, AUD0DAT+custom   ; max positive sample

This only produces a single sample word — not practical for continuous audio but useful for one-shot clicks/beeps.

audio.device

AmigaOS provides audio.device for arbitrated, multi-application audio access:

• Allocates channels (bitmask: {1,2,4,8})
• Creates IOAudio request, uses standard device I/O (BeginIO/WaitIO)
• Handles period, volume, sample pointer, cycle count
• See 10_devices/audio_device.md for full API reference

References

• ADCD 2.1 Hardware Manual — Paula audio chapter
• NDK39: hardware/custom.h (struct AudChannel), devices/audio.h
• Autodocs: audio.device — http://amigadev.elowar.com/read/ADCD_2.1/Includes_and_Autodocs_3._guide/node0081.html