Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!husc6!wjh12!redsox!campbell
From: campbell@redsox.bsw.com (Larry Campbell)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why is DOS limited to 640K?
Message-ID: <1464@redsox.bsw.com>
Date: 15 Oct 89 22:36:43 GMT
References: <8909270503.AA28536@euler.Berkeley.EDU> <10253@cbnews.ATT.COM>
Reply-To: campbell@redsox.UUCP (Larry Campbell)
Organization: The Boston Software Works, Inc.
Lines: 15

Generic MS-DOS is actually limited to 1 megabyte.  This is because
the entire MS-DOS "architecture" assumes that you're on an 8088, which
has a 20-bit address space.

PC-DOS -- that is, MS-DOS as implemented by IBM on the PC -- is limited
to 640K because IBM reserved the addresses above 640K for device adapters
and the ROM BIOS.

The DEC Rainbow (don't laugh!) supports up to 896K RAM, since it reserves
only the top 128K of the address space.  (No expansion slots, no device
adapters to worry about.)
-- 
Larry Campbell                          The Boston Software Works, Inc.
campbell@bsw.com                        120 Fulton Street
wjh12!redsox!campbell                   Boston, MA 02146