Intel® 82854 Graphics Memory Controller Hub (GMCH)

As an example, consider a BIOS that is implemented on the Expansion bus. During the initialization process, the BIOS can be shadowed in main system memory to increase the system performance. When BIOS is shadowed in main system memory, it should be copied to the same address location. To shadow the BIOS, the attributes for that address range should be set to Write Only. The BIOS is shadowed by first doing a Read of that address. This Read is forwarded to the Expansion bus. The Host then does a Write of the same address, which is directed to main system memory. After the BIOS is shadowed, the attributes for that system memory area are set to Read Only so that all Writes are forwarded to the Expansion bus. Figure 4 and Table 21 show the PAM registers and the associated attribute bits.

Figure 4. PAM Registers

P A M 6

P A M 5

P A M 4

P A M 3

P A M 2

P A M 1

P A M 0

Offset 5Fh 5Eh 5Dh 5Ch 5Bh 5Ah 59h

7

6

 

5

4

3

2

1

0

 

 

R

R

W E

R E

R

R

W E

RE

 

 

 

 

 

 

 

 

 

 

 

 

 

Reserved

 

 

 

 

 

 

 

 

Read Enable (R/W )

Reserved

 

 

 

 

 

 

 

 

1=Enable

W rite Enable (R/W)

 

 

 

 

 

 

0=Disable

 

 

 

 

 

 

 

 

1=Enable

 

 

 

 

 

 

 

Write Enable (R/W )

0=Disable

 

 

 

 

 

 

 

1=Enable

Read Enable (R/W )

 

 

 

 

0=Disable

 

 

 

 

 

 

 

1=Enable

Reserved

Reserved

0=Disable

 

 

pam

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D15343-003

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Intel D15343-003 manual PAM Registers