Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!uunet!mcsun!cernvax!chx400!ethz!forty2!eichi
From: eichi@forty2.UUCP (Stefan Eichenberger)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why is DOS limited to 640K?
Message-ID: <1111@forty2.UUCP>
Date: 7 Nov 89 16:18:00 GMT
References: <8909270503.AA28536@euler.Berkeley.EDU> <10253@cbnews.ATT.COM> <126@euteal.ele.tue.nl> <16320@dartvax.Dartmouth.EDU> <254F201F.22212@maccs.dcss.mcmaster.ca>
Reply-To: eichi@forty2.UUCP (Stefan Eichenberger)
Organization: Exp. Physics University Zuerich
Lines: 37

In article <254F201F.22212@maccs.dcss.mcmaster.ca> cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn) writes:

>1.  Find out how much RAM _really_ is there
>2.  Change the low-memory variable that says how much memory you have
>3.  Reboot the machine

This is done, for example by a program called MORERAM1.ARC on SIMTEL20
in <msdos.sysutl> (if I remember correct...).  

I asked the same question here about two weeks ago (how to make DOS aware
of more than 640 kBytes conventional memory) and got mail response to the
effect that it would be sufficient to modify the last MCB (memory control
block) such that the length is enlarged by the additional RAM. So, can any-
body tell me, why I should then go through the process of rebooting the 
machine? Modifying the last MCB is a program of about 20 Pascal lines and 
seems to work. Only INT 12 reads the above mentioned BIOS variable to
determine installed memory. So, what's wrong with

  1. Find out about how much RAM there is
  2. Patch BIOS data area at 0040:0013 to let BIOS know it (just in case...)
  3. Modify th last MCB
  4. Enjoy the memory

Are there any inconsistencies in this procedure? Any crashes to be expected?

Btw, rebooting the machine doesn't help with some BIOS clones such as the
AT&T6300 BIOS, which *always* (even at a warm reboot) rechecks the amount
of awailable memory (up to 640k only, of course) and resets 0040:0013! To 
overcome this problem I saw suggestgions such as patching the boot sector
of the hard disk...that's where the real dirty business starts!
 

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UUCP:      ...mcvax!cernvax!forty2!eichi         Stefan Eichenberger
BITNET:    K807817@CZHRZU1A                      University of Zurich
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