From: Temujin <temujin AT sentex DOT net>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Dos Version 6.20 and 6.21
Date: Mon, 25 Aug 1997 12:33:52 -0700
Organization: Sentex Communications
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References: <5to298$5js AT hyperion DOT triode DOT net DOT au>
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Andrew Happ wrote:
> 
> ========
> 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

This basically acomplishes nothing, if AL is moved into AH and then
zeroed out, the XCHG line is an extra useless line.

> 
> 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?