AM79C974 datasheet(79/153 Pages) AMD

79 / 153 page

P R E L I M I N A R Y

AMD

Am79C974

18681A/1-31

SMDLA

WMAC

MDL

Page Frame Address #1

Page Frame Address #2

Page Frame Address #3

Page Frame Address #4

Ignored

Page Frame Address #n

Ignored

In this example, the contents of the WMAC register is

pointing to page frame address #1. When the first entry

in the MDL is read (page frame address #1), the WMAC

(page frame address #2).

2. Issue the Start DMA Command. The SCSI control-

ler reads the page frame address (bits 31:12) from

the MDL entry and combines it with the first page

offset value in the Starting Physical Address (SPA)

the Working Address Counter (WAC) register and

becomes the physical address for page#1, as

shown below. The WMAC is then incremented to

point to the next entry in the MDL.

WAC

4K Page #1

Page Frame Address #1

Starting Offset

From the MDL

SPA

XXXX

Starting Offset

Programmed by

the software

Data

18681A/1-32

3. When the WAC register (bits 11:0) reaches the

first 4K byte boundary, the SCSI controller reads

the second MDL entry and combines the page

frame address (bits 31:12) from this entry of the

MDL with bits 11:00 of the WAC register. This be-

comes the physical address for page#2. Since the

WAC register (bits 11:0) has rolled over to ‘00h’,

the WAC now points to the beginning of the Page

Frame Address #2 as shown below. The WMAC

is then incremented to point to the next entry in

the MDL.

18681A/1-33

WAC

4K Page #2

Page Frame Address #2

From the MDL

Data

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