isA20on.Txt

Last Updated by Roedy Green 1993 June 8

Purpose
*******

isA20on is a method for diagnosing why some programs fail with a
"packed file corrupt message".  It will tell you if the A20
address line is enabled.

What it Does
************

Very high addresses can be interpreted two ways:

1.  they can reach above the 1 MB barrier into a small region of
    64K A20 memory called the HMA.  The CPU does this my enabling
    the A20 address line.

2.  they can wrap around to low ram.  The CPU does this by disabling
    the A20 address line.

Normally method 2 is used.  Programs treated with Microsoft's
ExePack require method 2 to be in effect, or you will get a
"packed file corrupt" message.

With DOS 5.0 and 6.0 the HMA became a popular place for DOS
itself.  DESQview also likes to use it.  So you may find the A20
line turned on by such programs.

For more detail, see the isA20on.ASM source code.

How To Use
**********

Run isA20on.Com

If it says that A20 is off, you are fine as is.

If it says that A20 is on, edit your advanced CMOS settings so
that "Enable CPUA20 to A20" is set to 1 to disable it.  If your
CMOS gives you no way to control the initial setting of the A20
enable, you can use MSFIX.COM to force programs to load above
the 64K boundary.  MSFIX is a kludge.  DOS 5.0 comes with a
similar kludge utility called LOADFIX.

To permanently fix the bug, you have to UNexepack the offending
program and pack it with something else like PkLite.
V-Communications makes such an UN-exepack called UNPACKER.

There is also a French free program called UpackExe.

Author
******

Please report bugs and problems to:

Roedy Green
Canadian Mind Products
#101 - 2536 Wark Street
Victoria, BC Canada V8T 4G8
tel:(250) 361-9093
mailto:roedyg@mindprod.com
http://mindprod.com

GETTING THE LATEST VERSION
**************************

Look for the latest version at my Web site:
  http://mindprod.com

It would also be helpful if you mentioned the URL or source of where
you got your copy.  I want to make sure that site is kept kept up to
date.

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