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From: dmt@mtuxt.UUCP (D.TUTELMAN)
Newsgroups: net.micro
Subject: Re: source of PC-DOS
Message-ID: <642@mtuxt.UUCP>
Date: Wed, 30-Apr-86 10:15:54 EDT
Article-I.D.: mtuxt.642
Posted: Wed Apr 30 10:15:54 1986
Date-Received: Fri, 2-May-86 08:41:42 EDT
References: <116@druri.UUCP>
Organization: AT&T Information Systems, Holmdel NJ
Lines: 41
To: dmt



[ Re: a request for (1) source code to PC-DOS, and (2) how
  DOS checks its memory limit as 640K. ]

I seriously doubt that Microsoft makes the source to PC-DOS
generally available.  I'm sure they treat it as the company
jewels; actually, that would be an appropriate business move 
on their part.

As for your other request, let me recommend some reading (along
with the answer).  If you're going to be doing much development
of PC software at a level indicated by your question, you ought
to read (or at least browse and understand):

   -	Peter Norton's excellent book, "Programmer's Guide to
	the IBM PC." Microsoft Press, 1985.

   -	The BIOS source for your computer.  Yup, it's in assembler,
	but it's probably pretty readable.  And there are several
	available, so you can see what's common and what's
	different.  IBM and AT&T both have source listings for
	their BIOS in their reference manuals, and DTC makes
	available the source for their MegaBIOS (I think I got
	the names right there).

Now the answer to your other question.  The ACTUAL amount of memory
is determined by the BIOS, in one of two ways.  Some machines read
the motherboard dipswitches and beleive what they say; others try
write-then-read probes at 16K or 64K intervals.  In either case,
the MAXIMUM amount they can find is hard-coded in the BIOS ROM.
In the case of expanded or extended memory add-on boards, some BIOS
will find it, but the BIOS & hardware will generally keep it out of
the 640K-1M region until an application wants to map it there.

             		Dave Tutelman
                	Physical - AT&T Information Systems
                  		Room 1H120
                   		Juniper Plaza, Route 9
                  		Freehold, NJ 07728
                	Logical -  ...ihnp4!mtuxo!mtuxt!dmt
             		Audible -  (201) 577 4232
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