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Theory of Operation
Memory Map
The Intel486 SL supports a 25-bit physical memory address. Memory at addresses
between 0 and the theoretical maximum of 20 MB (13FFFFFh) is mapped as follows:
Range
Content
0000000 - 009FFFF
DRAM (first 640 KB)
00A0000 - 00BFFFF
mapped to EXM expanison interface; almost
always used by a video controller as video RAM
00C0000 - 00CFFFF
Write-protected DRAM containing video BIOS
00D0000 - 00EFFFF
Uncommitted; mapped to EXM expansion
interface
00F0000 - 00FFFFF
Write-protected DRAM containing BIOS
0100000 - 13FFFFF
Extended memory when installed or
mapped to EXM expansion interface
1400000 - 1FEFFFF
Mapped to EXM expansion interface
1FF0000 - 1FFFFFF
Mapped to BIOS ROM
Table 5. Memory Map.
0D000 - 0EFFFF may be used either as page frame, BIOS extension, I/O buffer
(i.e. for extended memory managers, Ethernet, etc.) or may be used by DOS as upper
memory blocks if an extended memory manager driver is installed.
Note that since the EXM expansion interface has 24 address lines, some of the
"mapped to EXM expansion interface" address areas map repeatedly, or wrap-around,
in the expansion interface's address space.
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