Ref: 23230002
Title: Killer B's--Identifying Slow Ranges of High DOS Memory
date: 12/17/90

Copyright 3Com Corporation, 1991.  All rights reserved.

On 386 and 386SX machines, certain ranges of DOS high memory (640 to 1024 KB)
can be extremely slow, depending on the manufacturing quality.  The ranges
occur in the B000-C000 addresses.

To view the speeds of ranges in DOS high memory, use 3ComEMM.  Boot the 386
machine with the 3COMEMM.SYS device driver, then execute 3COMEMM.COM with
the /t parameter (found in \3UTIL on PC servers):

   3comemm /t

This program will execute a memory test, then display a table that looks
something like this:

3COMEMM  -- Version 2.26 (C) Copyright 1987, 1988 Qualitas, Inc.
+------------------------------------------------------------------------+
|               Timing memory accesses, please wait a moment...          |
|------------------------------------------------------------------------|
|                             MEMORY ACCESS TIMES                        |
|------------------------------------------------------------------------|
| Starting |     Range     |        | Average | Ratio to Fastest Time    |
| Address  | Start     End | Length | Time s | (1.0 = fastest)          |
|----------|---------------|--------|---------|--------------------------|
| 00000000 |     0 |   640 |   640  |    515  |  1.0                    |
| 000A0000 |   640 |   736 |    96  |   1057  |  2.1                 |
| 000B8000 |   736 |   748 |    12  |   2251  |  4.4           |
| 000BB000 |   748 |   752 |     4  |   2021  |  3.9           |
| 000BC000 |   752 |   768 |    16  |   2251  |  4.4           |
| 000C0000 |   768 |   896 |   128  |    515  |  1.0                    |
| 000E0000 |   896 |   960 |    64  |         |      Absent              |
| 000F0000 |   960 |  9336 |  8376  |    515  |  1.0                    |
| 0091E000 |  9336 |  9528 |   192  |         |      Absent              |
| 0094E000 |  9528 |  9600 |    72  |    515  |  1.0                    |
+------------------------------------------------------------------------+

The rightmost column shows the relative speeds of the memory in relation to
the first 640 KB of memory (this is the fastest).  Notice that several
address ranges are 4 times slower than the first 640 KB (usually video
memory).  Some clones have shown ranges of up to 10 times slower.  Be aware
that relocating LOCATOR or CIOSYS or any TRS to this area may degrade the
performance of those programs.  Also, if you need to move an EMS page frame
for some reason (such as 3C507 shared memory or a hard drive controller ROM
conflict), moving it to a slower area will decrease EMS application
performance (such as CIOSYS).
