Datasheet

Register Description
R
100 Datasheet
3.7.32 APSIZE – Aperture Size (Device #0)
Address Offset: B4h
Default Value: 00h
Access: Read/Write
Size: 8 bits
This register determines the effective size of the Graphics Aperture used for a particular
GMCH/MCH configuration. This register can be updated by the GMCH/MCH-specific BIOS
configuration sequence before the PCI standard bus enumeration sequence. 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.
Bit Description
7:6 Reserved
5:0 Graphics Aperture Size (APSIZE). Each bit in APSIZE[5:4] operates on similarly
ordered bits in APBASE[27:26] of the Aperture Base configuration register. When a
particular bit of this field is “0” it forces the similarly ordered bit in APBASE[27:26] to
behave as “0”. When a particular bit of this field is set to “1” it allows corresponding bit of
the APBASE[27:26] to be read/write accessible.
Only the following combinations are allowed when the Aperture is enabled:
Bits[5:4] Aperture Size
11 64 MB
10 128 MB
00 256 MB
Default for APSIZE[5:4]=00b forces default APBASE[27:26] =00b (i.e. all bits respond as
“hardwired” to 0). This provides maximum aperture size of 256 MB. As another example,
programming APSIZE[5:4]=11b enables APBASE[27:26] as read/write programmable
providing a minimum size of 64 MB.
3:0: Reserved set to zero for software compatibility.