isA20on 1.6 diagnoses the "packed file corrupt message".  It
will tell you if the A20 address line is enabled.  When A20 is
enabled, the 64K HMA region above 640K is accessible.
Unfortunately, because of a bug in EXEPACK, programs that were
treated with Exepack, that load into the first 64K of RAM, don't
work.  MASM/OPTAsm source included. By Roedy Green of Canadian
Mind Products.  Copyrighted but may be copied freely for
non-military use only.  Version 1.6 just embeds the new address
and phone number. This is only of interest for ancient DOS machines.