845GL datasheet(67/193 Pages) INTEL

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

3.5.1.21

FDHC—Fixed SDRAM Hole Control Register (Device 0)

Address Offset:

97h

Default Value:

00h

Access:

R/W, RO

Size:

8 bits

This 8-bit register controls a fixed SDRAM hole from 15 MB–16 MB.

3.5.1.22

SMRAM—System Management RAM Control Register (Device 0)

Address Offset:

9Dh

Default Value:

02h

Access:

R/W, RO, L

Size:

8 bits

The SMRAMC register controls how accesses to Compatible and Extended SMRAM spaces are

treated. The open, close, and lock bits function only when G_SMRAME bit is set to a 1. Also, the

open bit must be reset before the lock bit is set.

Bit

Description

Hole Enable (HEN).

This field enables a memory hole in SDRAM space. The SDRAM that lies

“behind” this space is not remapped.

0 = No memory hole

1 = Memory hole from 15 MB to 16 MB.

6:0

Reserved.

Bit

Description

Reserved.

SMM Space Open (D_OPEN)—R/W, L.

When D_OPEN=1 and D_LCK=0, the SMM space SDRAM

is made visible even when SMM decode is not active. This is intended to help BIOS initialize SMM

space. Software should ensure that D_OPEN=1 and D_CLS=1 are not set at the same time.

SMM Space Closed (D_CLS)—R/W.

When D_CLS = 1, SMM space SDRAM is not accessible to

data references, even if SMM decode is active. Code references may still access SMM space

SDRAM. This will allow SMM software to reference through SMM space to update the display even

when SMM is mapped over the VGA range. Software should ensure that D_OPEN=1 and D_CLS=1

are not set at the same time. Note that the D_CLS bit only applies to Compatible SMM space.

SMM Space Locked (D_LCK)—R/W, L.

When D_LCK is set to 1, D_OPEN is reset to 0; D_LCK,

D_OPEN, C_BASE_SEG, H_SMRAM_EN, TSEG_SZ and TSEG_EN become read only. D_LCK

can be set to 1 via a normal configuration space write but can only be cleared by a Full Reset. The

combination of D_LCK and D_OPEN provide convenience with security. The BIOS can use the

D_OPEN function to initialize SMM space and then use D_LCK to “lock down” SMM space in the

future so that no application software (or BIOS itself) can violate the integrity of SMM space, even if

the program has knowledge of the D_OPEN function.

Global SMRAM Enable (G_SMRARE)—R/W, L.

If set to a 1, Compatible SMRAM functions are

enabled, providing 128 KB of SDRAM accessible at the A0000h address while in SMM (ADS# with

SMM decode). To enable Extended SMRAM function this bit has be set to 1. Refer to Chapter 5 for

more details. Once D_LCK is set, this bit becomes read only.

2:0

Compatible SMM Space Base Segment (C_BASE_SEG)—R/W, L.

This field indicates the location

of SMM space. SMM SDRAM is not remapped. It is simply made visible if the conditions are right to

access SMM space, otherwise the access is forwarded to the hub interface. Since the GMCH

supports only the SMM space between A0000h and BFFFFh, this field is hardwired to 010.

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