AM79C974 datasheet(152/153 Pages) AMD

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AMD

A M E N D M E N T

Am79C974

When WAC (bits 11:0) again reaches the next 4K byte

boundary, the next MDL entry is read into the WAC. The

operation continues in this way until WMAC register

reaches the last MDL entry (Page Frame Address #n in

this example).

4. The WAC register points to the beginning of the last

page#n and the DMA operation continues until the

byte count is exhausted in the Working Byte

Counter (WBC) register. When WBC=0, the chip

stops incrementing the WAC register. This is

shown below.

18681A/1-34

WAC

4K Page #n

Page Frame Address #n

From the MDL

WBC = 0

Data

DMA Scatter-Gather Operation

(Non-4k aligned elements MDL not set)

There is another way to implement a scatter-gather op-

eration which does not force the data elements to be

aligned on 4k boundaries. It assumes a “traditional”

scatter-gather list of the following format:

Element 0

Physical Address Byte Count

Element 1

Physical Address Byte Count

...

Element n

Physical Address Byte Count

This second implementation is described as follows:

1. Set the SCSI Start Transfer Count Register

((B)+00h, (B)+04h, (B)+38h) to the Byte count of

the first Scatter-Gather element.

2. Program the DMA Starting Transfer Count Regis-

ter ((B)+44h) to the Byte Count of the first Scatter-

Gather element.

3. Program the DMA Starting Physical Address Reg-

ister ((B)+48h) to the Physical Address of the first

Scatter-Gather element.

4. Start the SCSI operation by issuing a SCSI Infor-

mation Transfer command.

5. Start the DMA Engine with DMA Transfer Interrupt

Enable (Bit 6, (B)+40h).

6. When the Scatter-Gather element’s Byte Count

is exhausted, the DMA engine will generate an

interrupt.

7. Reprogram the next Scatter-Gather element’s Byte

Count into the SCSI Start Transfer Count Register

and the DMA Starting Transfer Count Register.

8. Reprogram the DMA Starting Physical Address

next Scatter-Gather element.

9. Repeat steps 4–8 until the Scatter-Gather list is

completed.

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