Table 10-12.
Index 4DH----DRAM Configuration
Bit
Description
7
CAS
Wait State
Allows an additional T-state (two PROCCLK cycles) to be inserted on all
local memory accesses. The added T-state is inserted during the CAS
active interval, extending the width of the CAS pulse.
0 = no added Wait State (default)
1 = Wait State enabled
6
RAS Timeout
Disables the RAS Timeout feature.
0 = RAS timeout enable (default)
1 = RAS timeout disabled
5
CAS Wait State
for Read
Allows the CAS Wait State (bit 7 above) to be inserted on local memory
reads, but not on writes.
0 = Wait State controlled by bit 7 above (default)
1 = CAS Extend Wait State Enabled on memory read, regardless of
bit 7; wait state for write still controlled by bit 7.
4-0
CFG4-0
These bits specify the DRAM configuration as described in
Section 5,
System Interface,
subsection titled
DRAM Interface.
The default value
after reset is 00001 (512KB total DRAM).
Table 10-13.
Index 4EH----Extended Memory Boundary
Bit
Name
Description
7
RAS Encode Enable
This bit changes the four -RAS lines into three encoded -RAS lines and
one -RAS timing line. These four lines are used with an external 3-8
decoder to support eight banks of DRAM.
0 = -RAS encode disabled (default)
1 = -RAS encode enabled
6
MRA Enable
Enables the ‘‘Multiple RAS Active ’’ mode for higher system performance.
0 = Single RAS active mode (default)
1 = Multi RAS active mode. Also enables multiplexed DREQs
and encoded DACKs
5
RAM Disable
040000H-09FFFFH
This bit disables the 82C836’s internal DRAM controller for accesses in
the range. 040000H-09FFFFH. These accesses are directed to the I/O
channel.
0 = Enables 040000H-09FFFFH DRAM range (default).
1 = Disabled 040000H-09FFFFH DRAM range.
4
High ROM
Enable
This bit should be set to one for PC/AT compatibility (see also ICR 46H
bit 5, high shadow RAM enable).
0 = Enable FC0000H and above (256K) as ROM (default)
1 = Enable FE0000H and above (128K) as ROM.
82C386 CHIPSet Data Sheet
Configuration Registers
I
Chips and Technologies, Inc.
P R E L I M I N A R Y
Revision 3.0
10-9