Q74870: How EMM386.EXE Rounds Segment Addresses
Article: Q74870
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:5.x,6.0,6.2,6.21,6.22
Operating System(s):
Keyword(s):
Last Modified: 17-DEC-2000
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The information in this article applies to:
- Microsoft MS-DOS operating system versions 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22
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SUMMARY
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The version 5.0 "Microsoft MS-DOS User's Guide and Reference" states that when
using the X= and I= parameters with EMM386.EXE, addresses are rounded down to
the nearest 4K boundary. Specifically, the address range is rounded to the next
even 4K address closest to the hexidecimal address FFFFF.
MORE INFORMATION
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Because of this rounding procedure, it is possible that pieces of memory 1 to 4K
in size may not be correctly included or excluded by EMM386.EXE. The
significance of this rounding procedure becomes important when attempting to
arrange included and excluded memory blocks in a contiguous order. To ensure
that all pieces of memory are used correctly, use hexidecimal ranges that border
one place digit apart, rather than values that overlap.
For example, an address range E000-E100 overlaps one complete 4K piece of memory
by 16 bytes and is rounded upward toward FFFFF. This address range is actually
recognized as E000-E1FF, which is approximately 8K in size. In this example, to
specifically reference only the 4K piece of memory starting at E000, the correct
range is E000-E0FF.
The following chart demonstrates some examples of using include and exclude
parameters with EMM386.EXE and the NOEMS parameter. Remember that X= parameter
has priority over the I= parameter. The leftmost value in the chart is the total
amount of UMB memory reported when booting up. The K value is the rounded
kilobyte value given by the MEM/C command.
Total UMB X= and I= Parameters Result of MEM/C
Memory Showing Available
on Boot Up Bytes (K value)
---------- ------------------------------------- ------ ---------
3K X=A000-DFFF X=E100-FFFF I=E000-E0FF 4,064 4.0K
7K X=A000-DFFF X=E200-FFFF I=E000-E100 8,160 8.0K
3K X=A000-DFFF X=E100-FFFF I=E000-E100 4,064 4.0K
63K X=A000-DFFF I=E000-EFFF 65,504 64.0K
67K X=A000-DFFF I=E000-F000 69,600 68.0K
55K X=A000-DFFF X=E400-E500 I=E000-EFFF 57,248 55.9K
Key points from the above chart:
- Line 2 was intended to include only a 4K piece of memory but resulted in an
8K size.
- Line 3 is different from line 2 only by the second exclude parameter. Because
this exclusion range overlaps the inclusion range, it has priority and
restricts the size of the inclusion range.
- Line 5 was intended to include only a 64K piece of memory but actually
included 68K.
- Line 6 was intended to exclude only a 4K piece of memory but actually
excluded 8.1K.
Additional query words: 6.22 5.00 5.00a 6.00 6.20
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Keywords :
Technology : kbMSDOSSearch kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS500a
Version : MS-DOS:5.x,6.0,6.2,6.21,6.22
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