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Register Description
R
Intel
®
82845 MCH for SDR Datasheet 63
3.5.23 SMRAM—System Management RAM Control Register
(Device 0)
Address Offset: 9Dh
Default Value: 02h
Access: R/W, RO, R/W/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 the G_SMRAME bit is set to a 1.
Also, the OPEN bit must be reset before the LOCK bit is set.
Bit Description
7 Reserved.
6 SMM Space Open (D_OPEN)—R/W/L. When D_OPEN=1 and D_LCK=0, the SMM space
DRAM 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. When D_LCK is set to a 1, D_OPEN is reset to 0 and becomes read only.
5 SMM Space Closed (D_CLS)—R/W. When D_CLS = 1, SMM space DRAM is not accessible
to data references, even if SMM decode is active. Code references may still access SMM
space DRAM. This allows 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, 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 “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.
3 Global SMRAM Enable (G_SMRAME)—R/W/L.
0 = Disable
1 = Enable. Compatible SMRAM functions are enabled, providing 128 KB of DRAM 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.
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 DRAM” 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.
010 = Hardwired to 010 to indicate that the MCH supports the SMM space at A0000h–BFFFFh.