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Suggested Use of a PCI Window
This mem_cs_l range in Figure 1–23 is subdivided into several portions (such as the
BIOS areas) that are individually enabled/disabled using CSRs as listed here:
•
The MCSTOM (top of memory) register has a 2MB granularity and can be pro-
grammed to select the regions from lMB up to 512MB.
•
The MCSTOH (top of hole) and MCSBOH (bottom of hole) registers define a mem-
ory hole region where mem_cs_l is not selected. The granularity of the hole is 64KB.
•
The MARl,2,3 registers enable various BIOS regions.
•
The MCSCON (control) register enables the mem_cs_l decode logic, and in
addition selects a number of regions (0KB to 512KB).
•
The VGA memory hole region never asserts mem_cs_l.
mem_cs_l
Figure 1–23
4GB
512MB Max
16MB
Main Memory Hole
1MB
1MB-64KB
VGA Memory
(A0000-BFFF)
512KB
System Address Space
A–46
Decode Area
Hole
BIOS Area
Hole
MCSTOM
MCSTOH
MCSBOH
MCSCON
MAR1,2,3
MCSCON
MCSCON
LJ-04279.AI4
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