Intel 386 User Manual page 172

exactly as if they represented another address line. The following options are supported by the
chip select unit:
CASMM
0
1
X
To see how this extension of the CSU supports the SMRAM requirements, consider an embedded
system which has 1 Mbyte of 16-bit wide EPROM in the region 03F00000H to 03FFFFFFH and
1 Mbyte of 16-bit wide RAM in the region 00000000H to 000FFFFFH. A single 32 Kbyte RAM
in the region 00038000H to 0003FFFFH is added to support SMM. The chip selects for this sys-
tem during normal operation would be programmed as follows:
REGION CA25:11
EPROM
11 1111 0000 0000 0
RAM
00 0000 0000 0000 0
SMRAM
00 0000 0011 1000 0
Each row in the above table represents a region of memory and its associated chip select logic.
During initialization, these same chip selects could be programmed as follows:
REGION CA25:11
EPROM
11 1111 0000 0000 0
RAM
00 0000 0000 0000 0
SMRAM
00 0001 0011 1000 0
Only the SMRAM row has been changed; the SMRAM chip select has been redirected to the re-
gion 013F800H to 013FFFFH and the CASMM bit has been cleared. This allows the initialization
software to set up the SMRAM without entering the SMM. Note that the external design of the
system must guarantee that an SMI# cannot occur while the SMRAM is being initialized.
If the SMM driver needs to access the memory shadowed under the SMRAM, the chip selects
can be reconfigured as follows:
REGION CA25:11
EPROM
11 1111 0000 0000 0
RAM
00 0001 0000 0000 0
SMRAM
00 0000 0011 1000 0
This leaves the SMRAM in place but moves the normal RAM into the partition 0100000H to
01FFFFFH. The CASMM bit is masked so that the RAM is selected independent of SMM.
CMSMM
0 During normal mode only
0 During SMM only
1 During normal mode or SMM
CM25:11
00 0000 1111 1111 1
00 0000 1111 1111 1
00 0000 0000 0111 1
CM25:11
00 0000 1111 1111 1
00 0000 1111 1111 1
00 0000 0000 0111 1
CM25:11
00 0000 1111 1111 1
00 0000 1111 1111 1
00 0000 0000 0111 1
SYSTEM MANAGEMENT MODE
Chip select active:
CASMM CMSMM
0 0
0 0
1 0
CASMM CMSMM
0 0
0 0
0 0
CASMM CMSMM
0 0
0 1
1 0
BS16
1
1
0
BS16
1
1
0
BS16
1
1
0
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