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From: john@xanth.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: New Dos from Microsoft
Message-ID: <842@xanth.UUCP>
Date: Fri, 17-Apr-87 08:36:09 EST
Article-I.D.: xanth.842
Posted: Fri Apr 17 08:36:09 1987
Date-Received: Sat, 18-Apr-87 09:40:59 EST
References: <705@imsvax.UUCP> <631@edge.UUCP> <1622@bnrmtv.UUCP> <225@homxb.UUCP>
Organization: Old Dominion University, Norfolk Va.
Lines: 20

In article <225@homxb.UUCP>, gemini@homxb.UUCP (Rick Richardson) writes:
> The way I heard it, protected mode is faster than real.  I remember a
> conversation I had with one of the Venix (Unix) OS developers, and
> he said that when they went from real mode to protected mode Venix on
> the PC/AT, context switches were 30% faster.

Yes, context switching will be much faster in protected mode, since a
protected mode context switch will only involve changing a few words
in descriptor tables and the like, while a real mode context switch
would involve a decent amount of copying, especially with a straight
UNIX kernel port, which assumes that "u" (for user) area associated
with each process is at a fixed location.  With real mode, you copy
the process' u areas into and out of the fixed u area; with protected
mode, you just change the segment descriptor....

Typically, though, other operations will be somewhat slower.
-- 
John Owens		Old Dominion University - Norfolk, Virginia, USA
john@ODU.EDU		old arpa: john%odu.edu@RELAY.CS.NET
+1 804 440 4529		old uucp: {seismo,harvard,sun,hoptoad}!xanth!john