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Register Description
R
Intel
®
82845 MCH for SDR Datasheet 69
3.5.29 APSIZE—Aperture Size (Device 0)
Address Offset: B4h
Default Value: 00h
Access: R/W
Size: 8 bits
This register determines the effective size of the Graphics Aperture used for a particular MCH
configuration. This register can be updated by the MCH specific BIOS configuration sequence
before the PCI standard bus enumeration sequence takes place. If the register is not updated, the
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.
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. Only the following combinations are allowed:
5 4 3 2 1 0 Aperture Size
1 1 1 1 1 1 4 MB
1 1 1 1 1 0 8 MB
1 1 1 1 0 0 16 MB
1 1 1 0 0 0 32 MB
1 1 0 0 0 0 64 MB
1 0 0 0 0 0 128 MB
0 0 0 0 0 0 256 MB
Default for APSIZE[5:0]=000000b forces default APBASE[27:22] =000000b (i.e., all bits respond
as “hardwired” to 0). This provides maximum aperture size of 256 MB. As another example,
programming APSIZE[5:0]=111000b hardwires APBASE[24:22]=000b and while enabling
APBASE[27:25] as read/write.