Message-Id: <199604101807.OAA30152@mail-e2b-service.gnn.com>
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Date: Wed, 10 Apr 1996 14:12:51
From: Jethro Wright <Jetman AT gnn DOT com>
To: djgpp AT delorie DOT com
Subject: Re: Paths and .COM files

>As I understand it (and don't quote me here), .COM files don't get
>copies of the environment to work with.  They are also supposed to be
>limited in the things they can do, but it's supposedly a tradeoff for 
>reduced size.
>
>Am I right?  Wrong?  Crazy?  :)
>

	John:  The principal diff bet .EXEs and .COMs is that a .COM
is a simple memory image and an .EXE is a multi-segment image.  .COMs
generally can do everything that an .EXE can do, it's just that, by
default, you only start w/ a max of 64K of mem.  That is, your code and
data for the *image loaded by DOS* must fit into a sgl stmt.  By 
defalt, CS == DS == ES == SS.  This is a remnant from DOS' origin as 
a clone of the 8080/Z80 OS called CP/M.  Now if one's a real bonehead, 
one can subvert this behavior in one's code by creating and usg multiple 
segs, doing custom overlays, etc.  But who'd want to, given the ease 
w/ which one can create .EXEs.  Said another way:  there's almost no
reason to create .COM files anymore.

	However if, for test purps, you want to make a .COM file, here's
what you can do:

	1)  run DEBUG

	2)  use the Assemble command to enter in a few DOS ints or
	whatever you need to do

	3)  use the N cmd to name the .COM file, like NFOO.COM

	4)  I *believe* the next step is to set the CX reg, via
	the R cmd, w/ the nbr of bytes you just assembled

	5)  Finally use the W cmd, to write out the image

	I haven't done this is in years, so confirm the last two 
instructions by reading up on DEBUG usg HELP....   Jet


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