AM79C974 datasheet(67/153 Pages) AMD

67 / 153 page

P R E L I M I N A R Y

AMD

Am79C974

accept writes to the APROM offsets within the

Am79C974 I/O resources map. Startup code in the sys-

tem BIOS can perform the PCI configuration accesses,

the IESRWE bit write, and the APROM writes.

Direct Access to the Microwire Interface

The user may directly access the Microwire port through

the EEPROM register, BCR19. This register contains

bits that can be used to control the Microwire interface

pins. By performing an appropriate sequence of I/O ac-

cesses to BCR19, the user can effectively write to and

read from the EEPROM. This feature may be used by a

system configuration utility to program hardware con-

figuration information into the EEPROM.

EEPROM-Programmable Registers

The following registers contain configuration informa-

tion that will be programmed automatically during the

EEPROM read operation:

1. I/O offsets 0h – Fh

APROM locations

2. BCR2

Miscellaneous Configuration

3. BCR16

not used in the Am79C974

controller

4. BCR17

not used in the Am79C974

controller

5. BCR18

Burst Size and Bus Control

6. BCR21

Not Used

If the PREAD bit (BCR19) is reset to ZERO and the

PVALID bit (BCR19) is reset to ZERO, then the

EEPROM read has experienced a failure and the con-

tents of the EEPROM programmable BCR register will

be set to default H_RESET values. The content of the

APROM locations, however, will not be cleared.

Note that accesses to the APROM I/O locations do not

directly access the Address EEPROM itself. Instead,

these accesses are routed to a set of shadow registers

on board the Am79C974 controller that are loaded with

a copy of the EEPROM contents during the automatic

read operation that immediately follows the H_RESET

operation.

AM79C974 Datasheet(PDF) 67 Page - Advanced Micro Devices

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