845GL datasheet(143/193 Pages) INTEL

143 / 193 page

Intel® 82845G/82845GL/82845GV GMCH Datasheet

143

System Address

5.1.2

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 GMCH. The Top of Memory (TOM) is limited to 2 GB SDRAM. All

accesses to addresses within this range will be forwarded by the GMCH 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 GMCH provides a maximum SDRAM address decode space of 4 GB. The GMCH does not

remap APIC memory space. The GMCH does not limit SDRAM address space in hardware.

5.1.2.1

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.

5.1.2.2

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.

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