From: andyh AT triode DOT net DOT au (Andrew Happ)
Newsgroups: comp.os.msdos.djgpp
Subject: Dos Version 6.20 and 6.21
Date: Sun, 24 Aug 1997 00:16:22 GMT
Organization: Triode Internet
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Message-ID: <5to298$5js@hyperion.triode.net.au>
NNTP-Posting-Host: dialup-5.triode.net.au
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk

========
I've been looking at the difference between these 2 MSDOS releases.  A
file compare between the respective versions of IO.SYS and MSDOS.SYS
reveals no difference, although the build dates do vary.  Comparing
the  two COMMAND.COMs reveals identical code except for two op codes,
vis

DOS 6.20 uses
  XCHG   AH,AL
  XOR     AH,AH

DOS 6.21 uses
  XOR   AH,AH
  MOV   AL,15

Does any machine code buster or os guru know what this difference
means in terms of the command processor's operation?

I do understand that the promoted difference between these 2 versions
was the removal of the disk compression  functions that were present
in  6.20 but absent in 6.21.  The difference in COMMAND.COM is a
little more subtle.