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Register Description
R
50 Intel
®
82845 MCH for SDR Datasheet
3.5.10 APBASE—Aperture Base Configuration Register (Device 0)
Offset: 10–13h
Default: 0000_0008h
Access: R/W, RO
Size: 32 bits
The APBASE is a standard PCI Base Address register that is used to set the base of the Graphics
Aperture. The standard PCI Configuration mechanism defines the base address configuration
register such that only a fixed amount of space can be requested (dependent on which bits are
hardwired to 0 or behave as hardwired to 0). To allow for flexibility (of the aperture), an
additional register called APSIZE is used as a “back-end” register to control, which bits of the
APBASE will behave as hardwired to 0. This register will be programmed by the MCH specific
BIOS code that runs before any of the generic configuration software is run.
Note: Bit 9 of the MCHCFG register is used to prevent accesses to the aperture range before this
register
is initialized by the configuration software and the appropriate translation table structure has been
established in the system memory.
Bit Description
31:28 Upper Programmable Base Address—R/W. These bits are part of the aperture base set by
configuration software to locate the base address of the graphics aperture. They correspond to
bits [31:28] of the base address in the processor's address space that will cause a graphics
aperture translation to be inserted into the path of any memory read or write.
Default = 0000
27:22 Middle “Hardwired”/Programmable Base Address—R/W. These bits are part of the aperture
base set by configuration software to locate the base address of the graphics aperture. They
correspond to bits [27:4] of the base address in the processor's address space that will cause a
graphics aperture translation to be inserted into the path of any memory read or write. These
bits can behave as though they were hardwired to 0, if programmed to do so by the APSIZE bits
of the APSIZE register. This causes configuration software to understand that the granularity of
the graphics aperture base address is either finer or more coarse, depending on the bits set by
MCH-specific configuration software in APSIZE.
21:4 Lower “Hardwired” Base Address—RO. Hardwired to 0s. This forces a minimum aperture
size selected by this register to be 4 MB.
3 Prefetchable—RO. This bit is hardwired to 1 to identify the Graphics Aperture range as
prefetchable as per the PCI Local Bus Specification for the base address registers.
There are no side effects on reads, the device returns all bytes on reads, regardless of the byte
enables, and the MCH 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.