Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!microsoft!kevinro
From: kevinro@microsoft.UUCP (Kevin ROSS)
Newsgroups: comp.os.os2
Subject: Re: overlays in 32-bit OS/2
Keywords: Virtual Memory 80386
Message-ID: <54465@microsoft.UUCP>
Date: 4 May 90 21:03:33 GMT
References: <1990Apr27.155908.4260@geac.com> <273@caslon.cs.arizona.edu> <4401@plains.UUCP> <11790@portia.Stanford.EDU> <4416@plains.UUCP>
Reply-To: kevinro@microsoft.UUCP (Kevin ROSS)
Organization: Microsoft Corp., Redmond WA
Lines: 45

In article <4416@plains.UUCP> harlow@plains.UUCP (Jay B. Harlow) writes:
|In article <11790@portia.Stanford.EDU| dhinds@portia.Stanford.EDU (David Hinds) writes:
||    Because of Microsoft's choice of the unsegmented memory model, it is
||not possible for the OS to distinguish between user code and user data,
||
||    In my opinion, Microsoft made a mistake in adopting the flat model.
||The 80386 finally did segments right - to where they are useful and don't
||get in the way.  Segments support many more access control possibilities
||than pages.  It seems like a waste to ignore this functionality.

|I aggree with David, if Microsoft chose to 'COMPLETELY' drop segments
|(ie a FLAT memory model, NO SMALL, & NO SEGMENTS AT ALL) in 
|OS/2 2.0, It would be a waste/shame to bother with OS/2.
 
Why is it a waste? The functionality of segments just makes the world a
much more complicated place, not to mention slower. 


By dropping segments, we have accomplished 3 very important goals:


1) Done away with segment loads

   Segment loads while in protected mode are VERY expensive. It takes
   at least 22 clocks just to load a segment register. This has already
   proven to be a major speed win. 

2) Code size is smaller

   By using a flat memory model, you can use any of the registers to address
   any location in user memory. You don't need segment overrides use 
   es:si, or ds:bp. Compilers become much easier to write, and the 
   list goes on.


3) Made OS/2 programs portable to other architectures

   Someday in the not to distant future, OS/2 will run on other hardware
   platforms. By not messing around with segments, the source code will
   be very portable to other architectures. 


About the only true gain of keeping 32-bit segments would be the 
ability to allocate more that 4 gigabytes of memory. If you are 
using that much memory, you surely won't want to run on a 386!