82815 GMCH
R
62
Datasheet
3.4.21.
SMRAM—System Management RAM Control Register
(Device 0)
Address Offset:
70h
Default Value:
00h
Access:
Read/Write, Read-Only
Size:
8 bits
The SMRAM register controls how accesses to Compatible and Extended SMRAM spaces are treated,
and how much (if any) memory is “Stolen” from the system to support both SMRAM and graphics local
memory needs.
7
6
5
4
3
2
1
0
Graphics Mode Select
Upper SMM Select
Lower SMM Select
SMM
Space
Locked
E_SMRA
M_ERR
Bit
Description
7:6
Graphics Mode Select (GMS).
This field is used to enable/disable the Internal Graphics device and
select the amount of main memory that is “Stolen” to support the internal graphics device in VGA (non-
linear) mode only. These 2 bits only have meaning if we are not in AGP mode.
00 = Internal graphics device Disabled, No memory “Stolen”
01 = Internal graphics device Enabled, No memory “Stolen”
10 = Internal graphics device Enabled, 512 KB of memory “Stolen” for frame buffer.
11 = Internal graphics device Enabled, 1 MB of memory “Stolen” for frame buffer.
Notes:
When the internal graphics device is disabled (00), the graphics device and all of its memory and I/O
functions are disabled and the clocks to this logic are turned off; memory accesses to the VGA range
(A0000–BFFFF) are forwarded on to the hub interface and the graphics local memory space is NOT
“stolen” from main memory. Any change to the SMRAM register will not affect AGP mode or cause
the controller to go into AGP mode. When this field is non-zero, the internal graphics device and all of
its memory and I/O functions are enabled; all non-SMM memory accesses to the VGA range will be
handled internally and the selected amount of graphics local memory space (0, 512 KB or 1 MB) is
“stolen” from the main memory. Graphics memory is “stolen” AFTER TSEG memory is “stolen”.
Once D_LCK is set, these bits becomes read-only.
GMCH does not support VGA on local memory. Software must not use the 01 mode for VGA.