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Intel
82865G/82865GV GMCH Datasheet
System Address Map
4.3.2
Pre-Allocated Memory
Voids of physical addresses that are not accessible as general system memory and reside within the
system memory address range (< TOSM) 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.
It is the responsibility of BIOS to properly initialize these regions
.
Table 16
details the location and attributes of the regions. Enabling/Disabling these ranges are
described in the GMCH Control (GC) Register in Device 0.
Extended SMRAM Address Range (HSEG and TSEG)
The HSEG and TSEG SMM transaction address spaces reside in this extended memory area.
HSEG
SMM-mode processor accesses to enabled HSEG are remapped to 000A0000h–000BFFFFh. Non-
SMM-mode processor accesses to enabled HSEG are considered invalid and are terminated
immediately on the FSB. The exceptions to this rule are Non-SMM-mode write back cycles that
are remapped to SMM space to maintain cache coherency. AGP and HI originated cycles to
enabled SMM space are not allowed. Physical SDRAM behind the HSEG transaction address is
not remapped and is not accessible.
TSEG
TSEG can be up to 1 MB in size and is the first block after the top of usable physical memory.
SMM-mode processor accesses to enabled TSEG access the physical SDRAM at the same address.
Non-SMM-mode processor accesses to enabled TSEG are considered invalid and are terminated
immediately on the FSB. The exceptions to this rule are Non-SMM-mode write back cycles that
are directed to the physical SMM space to maintain cache coherency. AGP and HI originated
cycles to enabled SMM space are not allowed.
The size of the SMRAM space is determined by the USMM value in the SMRAM register. When
the extended SMRAM space is enabled, non-SMM processor accesses and all other accesses in this
range are forwarded to the hub interface. When SMM is enabled, the amount of memory available
to the system is equal to the amount of physical SDRAM minus the value in the TSEG register.
Table 16. Pre-Allocated Memory
Memory Segments
Attributes
Comments
00000000h–03E7FFFFh
R/W
Available System Memory 62.5 MB
03E80000h–03EFFFFFh
SMM Mode Only - processor Reads
TSEG Address Range
03E80000h–03EFFFFFh
SMM Mode Only - processor Reads
TSEG Pre-allocated Memory
03F00000h– 03FFFFFFh
R/W
Pre-allocated Graphics VGA memory.
1 MB (or 512 K or 8 MB) when IGD is
enabled.