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From: ralf@b.gp.cs.cmu.edu.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: New Dos from Microsoft
Message-ID: <22@b.gp.cs.cmu.edu>
Date: Tue, 14-Apr-87 11:27:05 EST
Article-I.D.: b.22
Posted: Tue Apr 14 11:27:05 1987
Date-Received: Sat, 18-Apr-87 03:07:05 EST
References: <705@imsvax.UUCP> <631@edge.UUCP>
Organization: Carnegie-Mellon University, CS/RI
Lines: 52

In article <631@edge.UUCP> doug@edge.UUCP (Doug Pardee) writes:
>Another consideration:  if'n you put the '286 into protected mode, the
>CPU *itself* slows way down!  So you end up with a slow CPU running
>applications which have to do long, slow operating-system calls in order
>to accomplish a simple "store byte into video buffer".
>
>The '386 is perhaps even worse.  I am not surprised at published reports that
>beta sites for "MS-DOS 5.0" (as it was called at the time) found that
>a '386 machine running 5.0 was *slower* than a '286 machine running 3.x!
>It ain't a bug in the DOS, folks.
>
>-- Doug Pardee -- Edge Computer Corp. -- Scottsdale, Arizona

I can understand why the 386 would slow down in protected mode, but why should
the 286 slow way down?  The 386 potentially requires three memory accesses
for each memory access in the program, due to virtual addresses being mapped
by pages rather than by segments as in the 286.  But why should the 286
require any additional memory accesses except when loading the segment
registers?  (on loading the segment registers, I imagine the 286 would
fetch the segment descriptor from the segment descriptor table; thus, the
only additional overhead would be during segment switching)

Also, a smart operating system would not require a system call to write to
the screen memory--it would simply set up a virtual screen (like TopView,
DESQview, etc) that the application can do with as it pleases, and update
the real screen memory every so often.

BTW: I have PC-Write running in a DESQview window.  PC-Write does as it
pleases with the virtual screen, and DESQview updates the real screen about
six times a second (== once every timeslice).

If I am wrong about the overhead of protected mode on the 286, please let
me know.  (flames welcome by email, this office tends to be chilly)

--
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| Snail: Ralf Brown                           --- Edsger Dijkstra             |
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-- 
+=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=+
| ARPA:  RALF@B.GP.CS.CMU.EDU            "Teaching COBOL ought to be          |
| AT&T:  (412) 268-3053 (school)          regarded as a criminal act"         |
| Snail: Ralf Brown                           --- Edsger Dijkstra             |
|        Computer Science Department                                          |
|        Carnegie-Mellon University      DISCLAIMER?  Who ever said I claimed |
|        Pittsburgh, PA 15213            anything?                            |
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