845GL datasheet(144/193 Pages) INTEL

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System Address

144

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

Table 5-2 details the location and attributes of the regions. Enabling/disabling these ranges are

described in the GMCH Control Register Device 0 (GC).

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 are terminated

immediately on the PSB. The exceptions to this rule are Non-SMM-mode Write Back cycles that

are remapped to SMM space to maintain cache coherency. AGP and hub interface-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 at the top of 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

PSB. 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 hub interface-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 5-2. Pre-allocated Memory

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 512 K or 8 MB) when IGD is

enabled.

03F80000h–03FFFFFFh

SMM Mode Only - processor

reads

TSEG Address Range

03F80000h–03FFFFFFh

SMM Mode Only - processor

reads

TSEG Pre-allocated Memory

845GL Datasheet(PDF) 144 Page - Intel Corporation

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