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From: brad@kontron.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: New Dos from Microsoft (actually 286 protected mode)
Message-ID: <1527@kontron.UUCP>
Date: Wed, 15-Apr-87 20:52:06 EST
Article-I.D.: kontron.1527
Posted: Wed Apr 15 20:52:06 1987
Date-Received: Sat, 18-Apr-87 01:08:48 EST
References: <1622@bnrmtv.UUCP>
Organization: Kontron Electronics, Mt. View, CA
Lines: 23

Glenn Connery writes:
...
> 
> 1) The 286 runs at least 50% FASTER in protected mode than in REAL mode.
>    If you think about it, this is obvious--REAL mode is a simulation.
...

Even a cursory look at Intel's iAPX 286 Programmer's Reference Manual
will show this to be false.

The simple facts about protected mode speed are:

  1.  Data manipulation instructions run at exactly the same speed
      in both real and protected modes.

  2.  Any instruction that alters a segment register (far jumps, calls,
      and returns, INT, IRET, LDS, LES) will be slower, as the new segment
      descriptors must be fetched and validated.  This also applies to
      response to external interrupts.

Brad Yearwood
Kontron Electronics  {voder, pyramid}!kontron!brad
Mountain View, CA