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82815 GMCH
R
Datasheet
49
31
26
25
24
16
Upper Prog. Base Address Bits
Lower
“HW”/Prog
Base
Address
Hardwired to 0s
15
4
3
2
0
Hardwired to 0s
Prefetch
able
Type
Mem
Space
Indicator
Bit
Description
31:26
Upper Programmable Base Address bits—R/W.
These bits are used to locate the range size selected
via lower bits 25:4.
Default = 0000
25
Lower “Hardwired”/Programmable Base Address bit .
This bit behaves as “hardwired” or as
programmable depending on the contents of the APSIZE register as defined below:
Aperture Size = 32 MB
#
r/w
Aperture Size = 64 MB
#
0 (default)
Bit 25 is controlled by the bit 3 of the APSIZE register in the following manner:
If bit APSIZE[3]=0 (indicating 64 MB aperture size), then APBASE[25]=0. If APSIZE[3]=1, then
APBASE[25]=r/w (read/write) allowing 32 MB aperture size if desired.
Default for APSIZE[3]=0b forces default APBASE[25] = 0b (bit responds as “hardwired” to 0). This
provides a default to the maximum aperture size of 64 MB. The GMCH specific BIOS is responsible
for selecting smaller size (if required) before PCI configuration software runs and establishes the
system address map.
24:4
Hardwired to 0.
This forces minimum aperture size selected by this register to be 32 MB.
3
Prefetchable—RO.
This bit is hardwired to 1 to identify the Graphics Aperture range as a prefetchable
(i.e., There are no side effects on reads, the device returns all bytes on reads regardless of the byte
enables, and the GMCH may merge processor writes into this range without causing errors).
2:1
Type—RO.
These bits determine addressing type and they are hardwired to 00 to indicate that address
range defined by the upper bits of this register can be located anywhere in the 32-bit address space.
0
Memory Space Indicator—RO.
Hardwired to 0 to identify aperture range as a memory range.