82845PE Datasheet, PDF (105/176 Pages) Intel Corporation – 82845GE Graphics and Memory Controller Hub (GMCH) and 82845PE Memory Controller Hub (MCH)

System Address
4.1.2
Extended Memory Area
This memory area covers 100000h (1 MB) to FFFFFFFFh (4 GB–1 Byte) address range and it is
divided into the following regions:
• Main System SDRAM Memory from 1 MB to the Top of Memory; maximum of 2 GB
SDRAM.
• AGP or PCI Memory space from the Top of Memory to 4 GB with two specific ranges:
• APIC Configuration Space from FEC0_0000h (4 GB–20 MB) to FECF_FFFFh and
FEE0_0000h to FEEF_FFFFh
• High BIOS area from 4 GB to 4 GB – 2 MB
Main System SDRAM Address Range (0010_0000h to Top of Main Memory)
The address range from 1 MB to the top of main memory is mapped to main SDRAM address
range controlled by the (G)MCH. The Top of Memory (TOM) is limited to 2 GB SDRAM. All
accesses to addresses within this range will be forwarded by the (G)MCH to the SDRAM unless a
hole in this range is created using the fixed hole as controlled by the FDHC register. Accesses
within this hole are forwarded to the hub interface.
The (G)MCH provides a maximum SDRAM address decode space of 4 GB. The (G)MCH does not
remap APIC memory space. The (G)MCH does not limit SDRAM address space in hardware.
4.1.2.1
15 MB–16 MB Window
A hole can be created at 15 MB–16 MB as controlled by the fixed hole enable (FDHC register) in
Device 0 space. Accesses within this hole are forwarded to the hub interface. The range of physical
SDRAM memory disabled by opening the hole is not remapped to the Top of the Memory – that
physical SDRAM space is not accessible. This 15 MB–16 MB hole is an optionally enabled ISA
hole. Video accelerators originally used this hole. It is also used by validation and customer SV
teams for some of their test cards. That is why it is being supported. There is no inherent BIOS
request for the 15 MB–16 MB hole.
4.1.2.2
Pre-Allocated Memory
Voids of physical addresses that are not accessible as general system memory and reside within
system memory address range (< TOM) are created for SMM-mode and legacy VGA graphics
compatibility. For VGA graphics compatibility, pre-allocated memory is only required in non-local
memory configurations.
Note: It is the responsibility of BIOS to properly initialize these regions.
Intel^® 82845GE/82845PE Datasheet
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