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78
Intel
82865G/82865GV GMCH Datasheet
Register Description
3.5.26
APSIZE—Aperture Size Register (Device 0)
Address Offset:
Default Value:
Access:
Size:
B4h
00h
RO, R/W
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; 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.
3.5.27
ATTBASE—Aperture Translation Table Register (Device 0)
Address Offset:
Default Value:
Access:
Size:
B8-BBh
00000000h
RO, R/W
32 bits
This register provides the starting address of the Graphics Aperture Translation Table Base located
in the main memory. 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 main memory address. The
ATTBASE register may be dynamically changed.
Bit
Descriptions
7:6
Reserved.
5:0
Graphics Aperture Size (APSIZE)—R/W.
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 the corresponding bit of the APBASE[27:22] to be
read/write accessible. 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
Descriptions
31:12
Aperture Translation Table Base (TTABLE)—R/W.
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 that it should be modified only when the GTLB has been disabled.
11:0
Reserved.