AC97

AC'97 is an audio codec standard developed by Intel Architecture Labs in 1997.

Detecting AC97 sound card

Sound card is connected into PCI with class 0x04 and subclass 0x01. If you found sound card you can check VendorID and DeviceID if it is supporting AC97. But, because today are used Intel HD Audio cards whose have subclass 0x03, you can probably expect every sound card with subclass 0x01 is sound card compatibile with AC97. AC97 use four BARs, but we need only BAR0 - Native Audio Mixer Base Address and BAR1 - Native Audio Bus Master Base Address. Important notice is that these BARs can be I/O or MMIO. Before any work with card, you must set bit 0 and bit 2 in PCI Command Register for right functionality.

Native Audio Mixer registers

Here is list of all useful registers of BAR0 base address.

Offset	Description	Length
0x00	Reset Register	word
0x02	Set Master Output Volume	word
0x0E	Set Microphone Volume	word
0x18	Set Output Volume of PCM patterns	word
0x1A	Select Input Device	word
0x1C	Set Input Gain	word
0x1E	Set Gain of Microphone	word
0x28	Supported extended functions	word
0x2A	Enabling extended functions	word
0x2C	Sample rate of front speaker	word

Here is description for all registers:

0x00 Reset Register

Writing any value info this register caused Register reset, what means that all NAM registers are changed into their default value.

0x02 Master Volume

Bit	Type	Name	Description
5:0	RW	Volume of right channel	Number*1,5=Volume in decibels
7:6	RO	Hardwired to 0
13:8	RW	Volume of left channel	Number*1,5=Volume in decibels
14	RO	Hardwired to 0
15	RW	Mute	1=Mute Sound

0 db means loudest output, 94,5 db means quietest output, mute bit set means no sound. Default value of this register is 0x8000.

0x0E Microphone Volume(gain)

Bit	Type	Name	Description
4:0	RW	Microphone gain	Every step mean 1,5 db, see below
5	RO	Hardwired to 0
6	RW	20 db	1=Add 20 db of volume(gain)
14:7	RO	Hardwired to 0
15	RW	Mute	1=Mute Sound

01000b means 0 db, so 00000b means 12 db, 11111b means -34,5 db and mute bit set means no sound. Default value of this register is 0x8008.

0x18 PCM output volume

Bit	Type	Name	Description
4:0	RW	Volume of right channel	Every step mean 1,5 db, see below
7:5	RO	Hardwired to 0
12:8	RW	Volume of left channel	Every step mean 1,5 db, see below
14:3	RO	Hardwired to 0
15	RW	Mute	1=Mute Sound

01000b means 0 db, so 00000b means 12 db, 11111b means -34,5 db and mute bit set means no sound. Default value of this register is 0x8808.

0x1A Select record input device

Only usable values of this register is 0x0000 what means inbuilt Microphone input and 0x0303 what means AUX input(for extern microphone).

0x1C Record Gain

Bit	Type	Name	Description
3:0	RW	Gain of right channel	Number*1,5=Volume in decibels
7:4	RO	Hardwired to 0
11:8	RW	Gain of left channel	Number*1,5=Volume in decibels
14:2	RO	Hardwired to 0
15	RW	Mute	1=Mute Sound

0 db means max gain, 22,5 db means quietest gain, mute bit set means no gain. Default value of this register is 0x8000. This register is for recording sound something like Master Volume is for playing sound.

0x1E Record Gain of Microphone

Bit	Type	Name	Description
3:0	RW	Gain of right channel	Number*1,5=Volume in decibels
7:4	RO	Hardwired to 0
11:8	RW	Gain of left channel	Number*1,5=Volume in decibels
14:2	RO	Hardwired to 0
15	RW	Mute	1=Mute Sound

0 db means max gain, 22,5 db means quietest gain, mute bit set means no gain. Default value of this register is 0x8000.

Extended capabilites registers

Some cards support more features like other sample rate than 48000. You have to check Extended capabilites register to check whose features card support, and next enable them by setting bytes in Enable capabilites register. There is no guarantee that sound cards always support some extended feature.

Native Audio Bus Master registers

Here is list of all useful registers of BAR1 base address.

Offset	Description	Length
0x00	NABM register box for PCM IN	below
0x10	NABM register box for PCM OUT	below
0x20	NABM register box for Microphone	below
0x2C	Global Control Register	dword
0x30	Global Status Register	dword

NABM register box contains these registers:

Offset	Description	Length
0x00	Physical Address of Buffer Descriptor List	dword
0x04	Number of Actual Processed Buffer Descriptor Entry	byte
0x05	Number of all Descriptor Entries	byte
0x06	Status of transferring Data	word
0x08	Number of transferred Samples in Actual Processed Entry	word
0x0A	Number of next processed Buffer Entry	byte
0x0B	Transfer Control	byte

Here is description for registers:

0x2C Global Control

Bit	Type	Name	Description
0	RW	Global Interrupt Enable(GIE)	0=Disable Interrupts 1=Enable Interrupts
1	RW	Cold reset	0=Device is in reset and can not be used 1=Resume to operational state
2	RW	Warm reset
3	RW	Shut down	0=Device is powered 1=Shut down
6:4	RW	(you do not need this bits)
19:7	RO	Reserved
21:20	RW	Channels for PCM Output	00=2 channels 01=4 channels 10=6 channels 11=Reserved
23:22	RW	PCM Output mode	00=16 bit samples 01=20 bit samples

0x30 Global Status

Bit	Type	Name	Description
21:20	RO	Channel capabilites	00=2 channels 01=4 channels 10=6 channels 11=Reserved
23:22	RO	Sample capabilites	01=16 and 20 bit samples are supported Other=Reserved, only 16 bit samples supported

This register contains much more bytes, but you probably do not need them.

0x00 0x10 0x20 Buffer Descriptor Base Address

Bit	Type	Name	Description
2:0	RO	Hardwired to 0
31:3	RW	Base Address	Physical base address of Buffer Descriptor List

0x04 0x14 0x24 Current Processed Entry

Bit	Type	Name	Description
4:0	RO	Current Entry	Number of Buffer Entry whose is actually processed
7:5	RO	Hardwired to 0

0x05 0x15 0x25 Last Valid Entry

Bit	Type	Name	Description
4:0	RW	Last Entry	Number of last Buffer Entry whose will be processed
7:5	RO	Hardwired to 0

0x06 0x16 0x26 Transfer Status

Bit	Type	Name	Description
0	RO	DMA controller status	0=Transferring data 1=Halted
1	RO	End of transfer	0=Current Entry not equal to Last Valid Entry 1=CE equal to LVE
2	R/WC	Last Buffer Entry transfered Interrupt	Software clears this bit writing 1 to it 1=Interrupt is firing
3	R/WC	IOC Interrupt	Software clears this bit writing 1 to it 1=Interrupt is firing
4	R/WC	Fifo ERROR Interrupt	Software clears this bit writing 1 to it 1=Interrupt is firing
15:5	RO	Reserved

If you get some interrupt, you must clear this register by write value 0x1C otherwise the interrupt will be fired forever.

0x08 0x18 0x28 Position in Current Entry

Bit	Type	Name	Description
15:0	RO	Number of transferred data	Number of samples whose was transfered from actual processed Entry

0x0A 0x1A 0x2A Prefetched Entry

Bit	Type	Name	Description
4:0	RO	Prefetched Entry	Number of prefetched Buffer Entry
7:5	RO	Hardwired to 0

0x0B 0x1B 0x2B Transfer Control

Bit	Type	Name	Description
0	RW	DMA controller control	0=Pause transferring 1=Transfer sound data
1	RW	Reset	0=Remove reset condition 1=Reset this NABM register box, this bit is cleared by card when is reset complete
2	RW	Last Buffer Entry Interrupt enable	0=Disable interrupt 1=Enable interrupt
3	RW	IOC Interrupt enable	0=Disable interrupt 1=Enable interrupt
4	RW	Fifo ERROR Interrupt enable	0=Disable interrupt 1=Enable interrupt
7:5	RO	Reserved

For IOC interrupt see below part Buffer Descriptor List.

Buffer Descriptor List

When you have sound data is memory, you must describe it in Buffer Descriptor List. Every entry of list have following format:

Offset	Description	Length
0x00	Physical Address to sound data in memory	dword
0x04	Number of samples in this buffer	word
0x06	Bit 15=Interrupt fired when data from this entry is transferred Bit 14=Last entry of buffer, stop playing Other bits=Reserved	word

Buffer Description List has 32 entries. Every entry can transfer a max of 0xFFFE samples, note that this means 0xFFFE samples and not 0xFFFE frames (A frame contains one sample for each channel), programming it with an odd number such as 0xFFFF is not allowed per the manual. If you have 2 channels 16 bit audio, value 0xFFFE means that (2 bytes per sample*0xFFFE)= just under 128 KB is transferred. Default are samples in 16 bit patterns, stored in 16 bit boxes, but AC97 can also support 20 bit quality. 20 bit pattern is stored in dwords in higher bytes. If entry have set bit 15(called IOC) after successful transfer of all data from entry is fired interrupt. If you want use it, be sure that IOCE bit in BUFFER_CNT register is set. If is set bit 14(called BUP) it means that this entry is last from this list and AC97 stop playing sound after transferring this entry. While playing sound, you have to roll over all 32 buffer entries.

Theory of Operation

During playing back, The BDL is a simple ring buffer between the producer (driver code running on the CPU) and the consumer (AC97's DMA).

The producer keeps updating the contents of the sample buffers described by the BDL entries and optionally the BDL itself, and the write pointer (Last Valid Entry register at offset 0x5 in the BOX).

The consumer keeps reading the BDL entries, the contents of the sample buffers, the read pointer (Current Processed Entry at offset 0x4 in the BOX), and a second read pointer (Prefetched Entry at offset 0xA in the BOX).

Unlike HDA's stream DMAs which can keep running in cycles by themselves, AC97's DMA will stop if its read pointer catches up with the write pointer.

Thus the driver code should take care to keep the sample buffers updated and the write pointer sufficiently ahead of the DMA's read pointer to avoid any underrun or DMA stoppage. The driver should also take note of the auxiliary read pointer if it needs to update BDL entries, as the DMA can prefetch BDL entries and possible read stale sample buffers if the driver updated the BDL entries after the prefetching.

Using AC97 sound card

In initalization of sound card you must resume card from cold reset and set power for it. It can be done by write value 0x2 to Global Control register if you do not want interrupts, or 0x3 if you want interrupts. After this, you should write any value to NAM reset register to set all NAM register to their defaults. After this, you can read card capability info from Global Status register what means how many channels it suport and if is 20 bit audio samples supported. Now is sound card ready to use.

Playing sound

Playing sound throught AC97 is easy as one, two, three:

• Set Master Volume and PCM output volume
• Load sound data to memory and describe it in Buffer Descriptor List
• Set reset bit of output channel(NABM transfer control register 0x1B, value 0x2) and wait for card will clear it
• Write physical position of BDL to NABM register 0x10(output BDL address register)
• Write number of last valid buffer entry to NABM register 0x15
• Set bit for transfering data - NABM transfer control register 0x1B, value 0x1

Now wait for interrupt if you use it, do not forget to clear status register by write value 0x1C to it, if you do not use interrupt, you can pool for bit 1 of NABM transfer status. When data are played, you have to fill next entries and update last valid entry.

Stop and resuming sound

Stopping sound is easy, you clear bit 0 in NABM transfer control register. If you want to resume playing, you can easy set this bit back. If you want absoultly stop sound or reprogram playing other sound, you must stop transferring data and after this reset this NABM register box(bit 1 in NABM transfer control register 0x1B). Resetting will set all registers in box to zero.

See Also

External Links

• Audio Codec '97 Specification
• Intel ICH7 HDA/AC'97 Programmer's Reference Manual
• Intel ICH5/ICH5R/ICH4 AC'97 Programmer's Reference Manual