PIIX datasheet(99/122 Pages) INTEL

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82371FB (PIIX) AND 82371SB (PIIX3)

Each PRD entry is 8 bytes in length. PRDs must be aligned on 64-Kbyte boundaries. The first 4 bytes specify

the byte address of a physical memory region. The next two bytes specify the count of the region in bytes

(64-Kbyte limit per region). A value of zero in these two bytes indicates 64 Kbytes. Bit 7 of the last byte

indicates the end of the table (EOT). Bus master operation terminates when the last descriptor has been

retired.

051910_3.drw

Byte 3

Byte 2

Byte 1

Byte 0

EOT

Reserved

Byte Count [15:1]

Memory Region Physical Base Address [31:1]

Memory

Region

Main Memory

051910

Figure 5. Physical Region Descriptor Table Entry

NOTE

The memory region specified by the descriptor cannot straddle a 64-Kbyte boundary. This means that

the byte count can be limited to 64 Kbytes and the incrementer for the current address register need

only extend from bit 1 to bit 15. Also, the total sum of the descriptor byte counts must be equal to or

greater than the size of the disk transfer request. If greater than the disk transfer request, the driver

must terminate the bus master transaction (by setting bit 0 in the Bus Master IDE Command Register

to 0) when the drive issues an interrupt to signal transfer completion.

Operation

To initiate a bus master transfer between memory and an IDE DMA slave device, the following steps are

required:

Software prepares a PRD Table in main memory. Each PRD is 8 bytes long and consists of an address

pointer to the starting address and the transfer count of the memory buffer to be transferred. In any given

PRD Table, two consecutive PRDs are offset by 8-bytes and are aligned on a 4-byte boundary.

Software provides the starting address of the PRD Table by loading the PRD Table Pointer Register . The

direction of the data transfer is specified by setting the Read/Write Control bit. Clear the Interrupt bit and

Error bit in the Status register.

Software issues the appropriate DMA transfer command to the disk device.

Engage the bus master function by writing a 1 to the Start bit in the Bus Master IDE Command Register

for the appropriate channel. The first entry in the PRD table is fetched by the PIIX. The channel remains

masked until the first descriptor is loaded.

The controller transfers data to/from memory responding to DMA requests from the IDE device.

At the end of the transfer, the IDE device signals an interrupt.

In response to the interrupt, software resets the Start/Stop bit in the command register. It then reads the

controller status and then the drive status to determine if the transfer completed successfully.

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