AMD SC3200 manual Physical Region Descriptor Format, Byte, Audio Data Buffer, Size

32581C Core Logic Module

Physical Region Descriptor Table Address

Before the bus master starts a master transfer it must be pro- grammed with a pointer (PRD Table Address register) to a Physical Region Descriptor Table. This pointer sets the start- ing memory location of the Physical Region Descriptors (PRDs). The PRDs describe the areas of memory that are used in the data transfer. The descriptor table entries must be aligned on a 32-byte boundary and the table cannot cross a 64 KB boundary in memory.

Physical Region Descriptor Format

Each physical memory region to be transferred is described by a Physical Region Descriptor (PRD) as illus- trated in Table 6-11.When the bus master is enabled (Command register bit 0 = 1), data transfer proceeds until each PRD in the PRD table has been transferred. The bus master does not cache PRDs.

The PRD table consists of two DWORDs. The first DWORD contains a 32-bit pointer to a buffer to be transferred. The second DWORD contains the size (16 bits) of the buffer and flags (EOT, EOP, JMP). The description of the flags are as follows:

•EOT bit - If set in a PRD, this bit indicates the last entry in the PRD table (bit 31). The last entry in a PRD table must have either the EOT bit or the JMP bit set. A PRD can not have both the JMP and EOT bits set.

•EOP bit - If set in a PRD and the bus master has completed the PRD’s transfer, the End of Page bit is set (Status register bit 0 = 1) and an SMI is generated. If a second EOP is reached due to the completion of another PRD before the End of Page bit is cleared, the Bus Master Error bit is set (Status register bit 1 = 1) and the bus master pauses. In this paused condition, reading the Status register clears both the Bus Master Error and the End of Page bits and the bus master continues.

•JMP bit - This PRD is special. If set, the Memory Region Physical Base Address is now the target address of the JMP. The target address must be on a 32-byte boundary so bits[4:0] must be written to 0. There is no data transfer with this PRD. This PRD allows the creation of a

looping mechanism. If a PRD table is created with the JMP bit set in the last PRD, the PRD table does not need a PRD with the EOT bit set. A PRD can not have both the JMP and EOT bits set.

Programming Model

The following discussion explains, in steps, how to initiate and maintain a bus master transfer between memory and an audio slave device.

In the steps listed below, the reference to “Example” refers to Figure 6-12 "PRD Table Example" on page 167.

1)Software creates a PRD table in system memory. Each PRD entry is 8 bytes long; consisting of a base address pointer and buffer size. The maximum data that can be transferred from a PRD entry is 64 KB. A PRD table must be aligned on a 32-byte boundary. The last PRD in a PRD table must have the EOT or JMP bit set.

Example - Assume the data is outbound. There are three PRDs in the example PRD table. The first two PRDs (PRD_1, PRD_2) have only the EOP bit set. The last PRD (PRD_3) has only the JMP bit set. This example creates a PRD loop.

2)Software loads the starting address of the PRD table by programming the PRD Table Address register.

Example - Program the PRD Table Address register with Address_3.

3)Software must fill the buffers pointed to by the PRDs with audio data. It is not absolutely necessary to fill the buffers; however, the buffer filling process must stay ahead of the buffer emptying. The simplest way to do this is by using the EOP flags to generate an SMI when a PRD is empty.

Example - Fill Audio Buffer_1 and Audio Buffer_2. The SMI generated by the EOP from the first PRD allows the software to refill Audio Buffer_1. The second SMI refills Audio Buffer_2. The third SMI refills Audio Buffer_1 and so on.

Table 6-11. Physical Region Descriptor Format

				Byte 3								Byte 2								Byte 1								Byte 0
DWORD	31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0								Memory Region Base Address									[31:1] (Audio Data Buffer)															0
1	E	E	J						Reserved														Size [15:1]									0
	O	O	M
	T	P	P

166 AMD Geode™ SC3200 Processor Data Book