82845MP Datasheet, PDF (75/157 Pages) Intel Corporation – Intel 845 Family Chipset-Mobile 82845MP/82845MZ Chipset Memory Controller Hub Mobile (MCH-M)

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Intel^® 82845MP/82845MZ Chipset-Mobile (MCH-M)
3.7.29.
APSIZE – Aperture Size – Device #0
Address Offset:
Default Value:
Access:
Size:
B4h
00h
Read/Write, Read Only
8 bits
This register determines the effective size of the Graphics Aperture used for a particular MCH-M
configuration. This register can be updated by the MCH-M 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.
Bit
Description
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] field to be read/write
accessible. Only the following combinations are allowed:
543210
Aperture Size
111111
4 MB
111110
8 MB
111100
16 MB
111000
32 MB
110000
64 MB
100000
128 MB
000000
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.
250687-002
Datasheet
75