82845GL datasheet(71/193 Pages) INTEL

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Intel® 82845G/82845GL/82845GV GMCH Datasheet

3.5.1.28

APSIZE—Aperture Size Register (Device 0)

Address Offset:

B4h

Default Value:

00h

Access:

RO, R/W

Size:

8 bits

This register determines the effective size of the Graphics Aperture used for a particular GMCH

configuration. This register can be updated by the GMCH-specific BIOS configuration sequence

before the PCI standard bus enumeration sequence takes place. If the register is not updated then a

default value will select an aperture of maximum size (i.e., 256 MB). The size of the table that will

correspond to a 256-MB aperture is not practical for most applications and therefore these bits

must be programmed to a smaller practical value that will force adequate address range to be

requested via APBASE register from the PCI configuration software.

3.5.1.29

ATTBASE—Aperture Translation Table Register (Device 0)

Address Offset:

B8–BBh

Default Value:

00000000h

Access:

Read Only, Read/Write

Size:

32 bits

This register provides the starting address of the Graphics Aperture Translation Table Base located

in the main SDRAM. This value is used by the GMCH’s Graphics Aperture address translation

logic (including the GTLB logic) to obtain the appropriate address translation entry required during

the translation of the aperture address into a corresponding physical SDRAM address. The

ATTBASE register may be dynamically changed.

Bit

Description

7:6

Reserved.

5:0

Graphics Aperture Size (APSIZE.

Each bit in APSIZE[5:0] operates on similarly ordered bits in

APBASE[27:22] of the Aperture Base configuration register. When a particular bit of this field is 0 it

forces the similarly ordered bit in APBASE[27:22] to behave as hardwired to 0. When a particular bit

of this field is set to 1 it allows corresponding bit of the APBASE[27:22] to be read/write accessible.

Default The default value (APSIZE[5:0]=000000b) forces the default APBASE[27:22] to read as

000000b (i.e. all bits respond as hardwired to 0). This provides the maximum aperture size of

256 MB. As another example, programming APSIZE[5:0] to 111000b hardwires APBASE[24:22] to

000b and enables APBASE[27:25] to be read/write programmable.

000000 = 256-MB Aperture Size

100000 = 128-MB Aperture Size

110000 = 64-MB Aperture Size

111000 = 32-MB Aperture Size

111100 = 16-MB Aperture Size

111110 = 8-MB Aperture Size

111111 = 4-MB Aperture Size

Bit

Description

31:12

Aperture Translation Table Base (TTABLE).

This field contains a pointer to the base of the

translation table used to map memory space addresses in the aperture range to addresses in main

memory.

NOTE:

This field should be modified only when the GTLB has been disabled.

11:0

Reserved.

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