Intel D15343-003 manual 1 15 MB-16 MB Window, Pre-allocated System Memory, Attributes Comments

Intel® 82854 GMCH System Address Map

5.4Main System Memory Address Range (0010_0000h to Top of Main Memory)

The address range from 1 MB to the top of main system memory is mapped to main DDR SDRAM address range controlled by the GMCH. The GMCH will forward all accesses to addresses within this range to the DDR 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 Hub interface.

The GMCH provides a maximum DDR SDRAM address decode space of 4-GB. The GMCH does not remap APIC memory space. The GMCH does not limit DDR SDRAM address space in hardware.

5.4.115 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 DDR SDRAM disabled by opening the hole is not remapped to the top of the memory – that physical DDR SDRAM space is not accessible. This 15 MB-16 MB hole is an optionally enabled ISA hole. Video accelerators originally used this hole. Validation and customer SV teams also use it for some of their test cards. That is why it is being supported. There is no inherent BIOS request for the 15-16 hole.

5.4.2Pre-allocated System 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. It is the responsibility of BIOS to properly initialize these regions. The number of UMA options has been extended. Allocation is at a fixed address in terms of rigid positioning of UMA system memory 'TOM-TSEG-UMA(size), but it is mapped at any available address by a PCI allocation algorithm. GMADR and MMADR are requested through BARs.

The following table details the location and attributes of the regions.

Table 27. Table 33. Pre-allocated System Memory

System Memory Segments	Attributes	Comments
00000000H - 03E7FFFFH	R/W	Available system memory 62.5 -MB
03E80000H - 03F7FFFFH	R/W	Pre-allocated Graphics VGA memory
		1-MB (or 4/8/16/32- MB) when IGD is
		enabled
03F80000H - 03FFFFFFH	SMM Mode Only - CPU Reads	TSEG Address Range
03F80000H - 03FFFFFFH	SMM Mode Only - CPU Reads	TSEG Pre-allocated system memory

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