82845G Datasheet, PDF (146/193 Pages) Intel Corporation – Intel 82845G/82845GL/82845GV Graphics and Memory Controller Hub (GMCH)

System Address
5.1.3
AGP Memory Address Ranges
The GMCH can be programmed to direct memory accesses to the AGP bus interface when
addresses are within either of two ranges specified via registers in the GMCH’s Device 1
configuration space. The first range is controlled via the Memory Base Register (MBASE) and
Memory Limit Register (MLIMIT) registers. The second range is controlled via the Prefetchable
Memory Base (PMBASE) and Prefetchable Memory Limit (PMLIMIT) registers.
Conceptually, address decoding for each range follows the same basic concept. The top 12 bits of
the respective Memory Base and Memory Limit registers correspond to address bits A[31:20] of a
memory address. For the purpose of address decoding, the GMCH assumes that address bits
A[19:0] of the memory base are zero and that address bits A[19:0] of the memory limit address are
FFFFFh. This forces each memory address range to be aligned to 1-MB boundary and to have a
size granularity of 1 MB.
The GMCH positively decodes memory accesses to AGP memory address space as defined by the
following equations:
• Memory_Base_Address ≤ Address ≤ Memory_Limit_Address
• Prefetchable_Memory_Base_Address ≤ Address ≤ Prefetchable_Memory_Limit_Address
The window size is programmed by the plug-and-play configuration software. The window size
depends on the size of memory claimed by the AGP device. Normally these ranges will reside
above the Top-of-Main-DRAM and below High BIOS and APIC address ranges. They normally
reside above the top of memory (TOM) so they do not steal any physical SDRAM memory space.
It is essential to support a separate Prefetchable range to apply the USWC attribute (from the
processor point of view) to that range. The USWC attribute is used by the processor for write
combining.
Note that the GMCH Device 1 memory range registers described above are used to allocate
memory address space for any devices sitting on AGP that require such a window. These devices
would include the AGP device, PCI-66 MHz/1.5 V agents, and multifunctional AGP devices where
one or more functions are implemented as PCI devices.
The PCICMD1 register can override the routing of memory accesses to AGP. In other words, the
Memory Access Enable bit must be set in the Device 1 PCICMD1 register to enable the memory
base/limit and prefetchable base/limit windows.
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Intel^® 82845G/82845GL/82845GV GMCH Datasheet