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82854 GMCH System Address Map
D15343-003 115
5.4 Main 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.
5.4.1 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 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.
5.4.2 Pre-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