CS5530A datasheet(113/259 Pages) NSC

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Functional Description (Continued)

Physical Region Descriptor Format

Each physical memory region to be transferred is

described by a Physical Region Descriptor (PRD) as illus-

trated in Table 3-59. 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.

ThePRD table consists oftwo DWORDs.The firstDWORD

contains a 32-bit pointer to a buffer to be transferred. This

pointer must be 16-byte aligned. The second DWORD con-

tains the size (16 bits) of the buffer and the EOT flag. The

size must be in multiples of 16 bytes. The EOT bit (bit 31)

must be set to indicate the last PRD in the PRD table.

Programming Model

The following steps explain how to initiate and maintain a

bus master transfer between memory and an IDE device.

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 4-byte boundary. The

last PRD in a PRD table must have the EOT bit set.

Software loads the starting address of the PRD table

by programming the PRD Table Address Register.

Software must fill the buffers pointed to by the PRDs

with IDE data.

Write 1 to the Bus Master Interrupt bit and Bus Master

Error (Status Register bits 2 and 1) to clear the bits.

Set the correct direction to the Read or Write Control

bit (Command Register bit 3).

Engage the bus master by writing a “1” to the Bus

Master Control bit (Command Register bit 0).

The bus master reads the PRD entry pointed to by the

PRD Table Address Register and increments the

address by 08h to point to the next PRD. The transfer

begins.

The bus master transfers data to/from memory

responding to bus master requests from the IDE

device. At the completion of each PRD, the bus mas-

ter’s next response depends on the settings of the

EOT flag in the PRD. If the EOT bit is set, then the IDE

bus master clears the Bus Master Active bit (Status

the transfer, the bus master sets the Bus Master Error

bit (Status Register bit 1).

Table 3-59. Physical Region Descriptor Format

DWORD

Byte 3

Byte 2

Byte 1

Byte 0

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10

876543

210

Memory Region Physical Base Address [31:4] (IDE Data Buffer)

Reserved

Size [15:4]

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