Path: utzoo!yunexus!torsqnt!dptcdc!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ukma!husc6!cfa!ward
From: ward@cfa.harvard.EDU (Steve Ward)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Problem of breaking DOS 640K RAM limit?
Summary: 640K limit and MSDOS
Message-ID: <1429@cfa.cfa.harvard.EDU>
Date: 8 Mar 89 18:40:50 GMT
Article-I.D.: cfa.1429
References: <74032HJW2@PSUVM> <15373@cup.portal.com> <1345@ccncsu.ColoState.EDU>
Organization: Harvard-Smithsonian
Lines: 48


MSDOS does NOT impose the 640K limit commonly encountered on IBM and
clone PC's.

The MSDOS upper memory limit is affected by three things:
1. BIOS
2. MEMORY HARDWARE
3. BIOS EXTENSION ROM's (usually on plug-in I/O cards)

First, the BIOS sizes memory and stores this information.  All the PC/XT
BIOS's I have seen are hard-coded to stop testing at 640K.  In order to
utilize memory that may exist beyond 640K the BIOS will have to be
modified to test/size the extra memory.  MSDOS makes a BIOS call (BIOS
int, can't remember the int vector number) to obtain the contiguous
memory size, so MSDOS is not hard-wired for any limitation at least
up to 1MB.  MSDOS may even accept a memory size beyond 1MB but I have
not bothered to look into this because my hardware won't allow it.

On the PC/XT (8088/8086) there is a 1MB memory address space.  However,
BIOS extension ROM's/EPROM's are commonly found starting at address
C0000h and above.  This means if you can modify your BIOS to size or
report more memory, you could at least go up to 768K bytes of memory
before running into the BIOS extension ROM's.  If you could relocate
your BIOS extension ROM's (or didn't have any, though that is unlikely)
then you might make even more room.  Of course, there are plug-in
memory boards that would allow you install more memory contiguous
with the motherboard 640K, so at least it is possible to add the memory.

The second item, memory hardware, is usually only a severe limitation
for cases where the motherboard has limited PC I/O bus slots and
adding a memory card is not possible.  Item three was just explained.

So, if you can modify your BIOS you can add at least 128K (must be
contiguous with the 640K on the motherboard) and bring your memory
up to 768K.  If you can relocate BIOS extension ROM's above C0000h
then you can free up even more memory.  Some I/O boards allow selecting
alternate address locations for their ROM's so this is a possibility.

A careful check of memory space use by all your I/O boards should be
made to make sure that any memory you install will not clash in the
memory address space.

I believe there are a few PC bus memory boards on the market which
are half-size and allow adding up to 384K of memory.  Such a board
could possibly be used to add memory above, but contiguous with the
640K.  Now all you to do is disassemble your BIOS chip and modify it!

Steve W.   ward@cfa.harvard.edu