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72
Intel
82865G/82865GV GMCH Datasheet
Register Description
3.5.20
SMRAM—System Management RAM Control Register
(Device 0)
Address Offset:
Default Value:
Access:
Size:
9Dh
02h
R/W, RO, Lock
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 the G_SMRAME bit is set to 1. Also, the
open bit must be reset before the lock bit is set.
Bit
Descriptions
7
Reserved.
6
SMM Space Open (D_OPEN)—R/W.
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.
5
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.
4
SMM Space Locked (D_LCK)—R/W.
When D_LCK is set to 1, then D_OPEN is reset to 0 and
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 “l(fā)ock 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.
3
Global SMRAM Enable (G_SMRARE)—R/W/L.
If set to 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 to be set to 1. Refer to the section
on SMM for more details. Once D_LCK is set, this bit becomes read only.
2:0
Compatible SMM Space Base Segment (C_BASE_SEG)—RO.
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 HI. Since the GMCH supports only the
SMM space between A0000h and BFFFFh, this field is hardwired to 010.