CSND Shared Memory

This page describes the structure of the CSND shared memory.

Type0 Command Structure

Index Byte	Size	Description
0x0	2	Offset in shared-memory for the command to process immediately after this one, when not 0xFFFF.
0x2	2	CommandID
0x4	1	This is initially zero when this command data is written by the user process, once the CSND module finishes processing the command this is set to 0x1. This flag is only set to value 1 for the first command(once processing for the entire command chain is finished) at the offset specified in the service command, not all type0 commands in the chain.
0x5	3	Padding?
0x8	4	The low 5-bits for this field is the the channel-index. This word is used for command parameters as well.
0xC	0x14	Command parameters

Type0 Commands

Command 0x0

Index Byte	Size	Description
0x2	2	CmdID 0x0
0xC	4	Value
0x10	0x10	Unused, all-zero.

Value=1 begins audio playback. Value=0 stops audio playback, and also seems to reset the audio channel CSND registers so that those registers need re-initialized before audio can be played from this channel again.

Command 0x1

Index Byte	Size	Description
0x2	2	CmdID 0x1
0xC	4	Value
0x10	0x10	Unused, all-zero.

This command starts playing the audio channel with value=1. Once the audio channel is playing, value=0 pauses the audio channel playback, and value=1 resumes the audio channel playback. Value=1 has no affect when the audio channel is already playing, likewise for value=0 when the audio channel is not playing.

Command 0x2

Index Byte	Size	Description
0x2	2	CmdID 0x2
0xC	1	Encoding, this is the same as the values used with command 0xE.
0x10	0x10	Unused, all-zero.

This sets the encoding field in the CSND channel CNT register(that field is cleared to zero first), command 0xE also sets this field.

Command 0x3

Index Byte	Size	Description
0x2	2	CmdID 0x3
0xC	4	Physical address of the second block of audio data
0x10	4	Total byte-size of the block

This sets the address and size of the second block of an audio channel. The CSND module writes these fields to the channel register for the physical address of the second block and the register for the block size.

Command 0x4

Index Byte	Size	Description
0x2	2	CmdID 0x4
0xC	1	Looping value, this is the same as the values used with command 0xE.
0x10	0x10	Unused, all-zero.

This is the same as cmd2 except for the looping field. CSND will only write to this channel CNT register for updating the looping field if bit15 in the channel CNT register is value zero.

Command 0x5

Index Byte	Size	Description
0x2	2	CmdID 0x5
0xC	4	Value
0x10	0x10	Unused, all-zero.

When value=0, CSND module will clear bit7 in the channel CNT register. Otherwise, CSND module sets bit7 in the channel CNT register.

Command 0x6

Index Byte	Size	Description
0x2	2	CmdID 0x6
0xC	4	Value
0x10	0x10	Unused, all-zero.

This is the same as cmd5, except this updates channel CNT bit6 instead. CSND module will only update CNT here if bit15=0 in CNT.

Command 0x7

Index Byte	Size	Description
0x2	2	CmdID 0x7
0xC	1	Duty value (3 bits), see below
0x10	0x10	Unused, all-zero.

This command updates bit0 through (at least) bit2 of the channel's CNT register. These bits control the duty of a PSG square wave:

 0  12.5% "_______-_______-_______-"
 1  25.0% "______--______--______--"
 2  37.5% "_____---_____---_____---"
 3  50.0% "____----____----____----"
 4  62.5% "___-----___-----___-----"
 5  75.0% "__------__------__------"
 6  87.5% "_-------_-------_-------"
 7   0.0% "________________________"

Command 0x8

Index Byte	Size	Description
0x2	2	CmdID 0x8
0xC	4	s32 value = (s32)(6.7027964E+07f / ((float)samplerate * 1.0f))

This sets the sample rate. This value is written to the channel CNT high-u16, which command 0xE also writes to. This command is used immediately after command 0xE.

Command 0x9

Index Byte	Size	Description
0x2	2	CmdID 0x9
0xC	2	Left channel volume
0xE	2	Right channel volume
0x10	2	Left capture volume
0x12	2	Right capture volume

This sets the playback and capture volumes for the specified channel. This command is usually used after command 0x8(user processes use cmd8 a second time before using this command).

Command 0xA

Index Byte	Size	Description
0x2	2	CmdID 0xA
0xC	4	Physical address of the first block of audio data
0x10	0x4	Total byte-size of the block
0x14	0xC	Unused, all-zero.

This sets the address and size of the first block of an audio channel. The CSND module writes these fields to the channel register for the physical address of the first block and the register for the block size.

Command 0xB

Index Byte	Size	Description
0x2	2	CmdID 0xB
0xC	2	User processes set this to: cwav_imaadpcminfo.data
0x10	1	User processes set this to: cwav_imaadpcminfo.tableindex
0x11	0xF	Unused, all-zero.

This command is used for setting the IMA-ADPCM state used to play the first block (when IMA-ADPCM encoding is used), prior to using command 0xE. CSND channelslotregister+0x18 = (u16)cmdword[3] | ((cmdword[4] & 0xFF)<<16). CSND channelslotregister+0x1C = 0.

Command 0xC

Index Byte	Size	Description
0x2	2	CmdID 0xC
0xC	2	User processes set this to: cwav_imaadpcminfo.loopdata
0x10	1	User processes set this to: cwav_imaadpcminfo.looptableindex
0x11	0xF	Unused, all-zero.

This is used for setting the IMA-ADPCM state used to play the second block (when IMA-ADPCM encoding is used). channelslotregister+0x1C = (1<<31) | (u16)cmdword[3] | ((cmdword[4] & 0xFF)<<16).

Command 0xD

Index Byte	Size	Description
0x2	2	CmdID 0xD
0xC	1	0 = disable ADPCM state reload for the second block, 1 = enable.

This command configures bit31 of channelslotregister+0x1C. This bit indicates whether the hardware will load ADPCM state from said register everytime the second block is to be played; or if it will continue using the same state used for the previously emitted sample.

Command 0xE

Index Byte	Size	Description
0x2	2	CmdID 0xE
0x8	4	See below. (Flags + Timer)
0xC	4	See below. (Channel volume)
0x10	4	See below. (Capture volume)
0x14	4	Physical address for the first block of audio data.
0x18	4	Physical address for the second block of audio data.
0x1C	4	Total byte-size of one block.

This initializes the CSND channel registers located at: 0x1EC03400 + (channel_index*0x20). It is unknown if stereo samples are supported. In manual mode, the hardware plays the audio data of the first block endlessly ignoring the size field. In one-shot mode, the hardware plays the first block with the specified size and then stops. When looping is enabled, the hardware plays the first block once and then repeats the second forever. Before playing a block, the hardware reads the corresponding address register in order to know where it is located. The block size is either read only once at the beginning (mode 3) or before each block (mode 1, normal looping).

Parameter word 0x8

Bit	Description
5-0	Channel index
6	0 = disable linear interpolation, 1 = enable linear interpolation.
9-7	Ignored
11-10	Repeat mode: 0 = manual, 1 = normal looping, 2 = one-shot, 3 = looping with constant block size.
13-12	This sets the encoding, some of the names for the following encodings are from here. 0 = PCM8, 1 = PCM16, 2 = IMA-ADPCM. 3 = PSG, similar to DS.
14	0 = disable playback, 1 = enable playback
15	Ignored
31-16	Sample rate clamped to the following range(by the user process): 0x42-0xFFFF. This is the same value used with Cmd 0x8, except clamped.

Bits 0 through 5 are written as zero to the CNT register.

PSG: Channels 8 through 13 play a square tone that is 32 samples long. Channels 14 and 15 play noise. The duty of the square tones is set using command 0x7.

Parameter word 0xC

Bit	Description
15-0	Left channel volume, this value is clamped to the following range(by the user process): 0x0-0x8000.
31-16	Right channel volume, this value is clamped to the following range(by the user process): 0x0-0x8000.

Parameter word 0x10

Bit	Description
15-0	Left capture volume, this value is clamped to the following range(by the user process): 0x0-0x8000.
31-16	Right capture volume, this value is clamped to the following range(by the user process): 0x0-0x8000.

Command 0xF

Index Byte	Size	Description
0x2	2	CmdID 0xF
0x8	4	Flags, as in command 0xE. Only the timer and the start/stop (bit14) bits are used.
0xC	4	Channel volume, same as in command 0xE.
0x10	4	Capture volume, same as in command 0xE.
0x14	4	Duty (see command 0x7).

This channel is similar to command 0xE, but it is tailored for configuring PSG Tone channels (8, 9, 10, 11, 12, 13) as it contains a duty parameter. Channel format is set to PSG.

Command 0x10

Index Byte	Size	Description
0x2	2	CmdID 0xF
0x8	4	Flags, as in command 0xE. Only the timer and the start/stop (bit14) bits are used. Channel format is set to PSG.
0xC	4	Channel volume, same as in command 0xE.
0x10	4	Capture volume, same as in command 0xE.

This channel is identical to command 0xF except for not having a duty parameter. Presumably used to configure PSG Noise channels (14, 15).

Command 0x100

Writes bit15 of (u16*)(0x1EC03800+16*X).

Command 0x101

Writes bit0 of (u16*)(0x1EC03800+16*X).

Command 0x102

Writes bit1 of (u16*)(0x1EC03800+16*X).

Command 0x103

Writes bit2 of (u16*)(0x1EC03800+16*X).

Command 0x104

Sets (u16*)(0x1EC03800+16*X+4) = -Y.

Command 0x105

Sets (u32*)(0x1EC03800+16*X+8) = Y, and (u32*)(0x1EC03800+16*X+12) = Z.

Command 0x106

Does the same thing as all 0x100-0x106 combined.

Command 0x200

Sends some interrupt to the DSP?

Command 0x300

Index Byte	Size	Description
0x2	2	CmdID 0x300
0x8	0x18	Unused, all-zero.

CSND module writes state info(including state from CSND registers) for each audio channel to shared-memory for this command.

Offset1

This section describes the structure of the data located at offset1(from CSND:Initialize) in shared-memory. This data is updated by command 0x300, and by commands which update the channel physical address registers. The following describes the format of the entries located here.

Entry Format

Index Byte	Size	Description
0x0	1	0 = audio channel is not playing, 1 = audio channel is playing. CSND module sets this field to zero when bit14-bit15 in the channel CNT register are all-zero, otherwise CSND module sets this to value 1.
0x1	3	Padding
0x4	2	This is from the low 16-bits from chanregisterslot+0x18(see type0 cmd 0xB).
0x6	1	Value = (*(chanregisterslot+0x18) & 0x7F0000) >> 16(see type0 cmd 0xB).
0x7	1	Padding
0x8	4	This is the physical address from the main physical address channel-slot register. When the audio channel is playing, this is the address of the sample-data that was being played at the time this address was written here by CSND module.