Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!bu-cs!mirror!prism!rob
From: rob@prism.TMC.COM
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why is DOS limited to 640K?
Message-ID: <206900131@prism>
Date: 16 Oct 89 18:18:00 GMT
References: <10253@cbnews.ATT.COM>
Lines: 16
Nf-ID: #R:cbnews.ATT.COM:-1025300:prism:206900131:000:791
Nf-From: prism.TMC.COM!rob    Oct 16 14:18:00 1989


    There are basically two reasons for the 640K limitation. First, DOS
was designed to run on the Intel 8088, which is limited to addressing
1Mb of memory. When running on newer Intel CPUs (80286, 386, etc.), DOS
runs them in 8088 emulation (or 'real') mode, so that they are bound by
the same 1Mb constraint.

   Due to the design of most PC's, only the first 640K of that 1Mb can
be used by DOS. The remaining area is reserved for such things as BIOS
code, video buffers, and so on. 

   It's possible to make a larger memory available to DOS by remapping
the memory used by the BIOS or the video adapter. But there are serious 
difficulties with this approach (for example, most PC programs make 
assumptions about the address of the video buffer), and relatively few
PC vendors do this.