Xref: utzoo comp.sys.ibm.pc.programmer:422 comp.sys.ibm.pc:46548
Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!mcgill-vision!bloom-beacon!snorkelwacker!usc!cs.utexas.edu!wuarchive!udel!rochester!pt.cs.cmu.edu!b.gp.cs.cmu.edu!Ralf.Brown@B.GP.CS.CMU.EDU
From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.sys.ibm.pc.programmer,comp.sys.ibm.pc
Subject: Re: How is a 68000 as fast as an 80386??
Message-ID: <25fe3a99@ralf>
Date: 14 Mar 90 11:35:37 GMT
Sender: ralf@b.gp.cs.cmu.edu
Followup-To: comp.sys.ibm.pc.programmer
Organization: Carnegie Mellon University School of Computer Science
Lines: 43
In-Reply-To: <908@tijc02.UUCP>

In article <908@tijc02.UUCP>, rdo031@tijc02.UUCP (Rick Odle           ) wrote:
}From article <3666@plains.UUCP>, by harlow@plains.UUCP (Jay B. Harlow):
}> In article <505@bilver.UUCP> alex@bilver.UUCP (Alex Matulich) writes:
}>>Naturally, I wanted more speed, so I ported the program to an AT&T 386WGS
}>>at work, which is a 25 MHz 80386 IBM compatible.  I compiled it using
}>>Turbo C 2.0, large memory model.  Then I watched in chagrined disbelief as
}>>that number-crunching section still took about a minute to execute --
}>>actually a few seconds longer than my Amiga.  All source code was the same!
}>>
}>>5)  I know the 80386 has special modes of operation, incompatible with
}>>    previous chips, that allow it to run at its full potential.  Is this
}>>    the reason my program isn't running at its rightful speed?  Are these
}>       Yes partly, 1) 32-bit inst in real mode 2) 32-bit Protected mode
}>>    special modes accessible when using DOS?  If so, how?
}> 
}> Alex,
}>    the reason your program ran 'slow' on the 386 is because you compiled it
}> for a 286 (at best) LARGE model, a large model on a x86 means all addresses
}> have segment offset, haveing the x86 load segments is very 'expensive'
}
}The only fair test here is to do the test with large model.  While it
}is true that the 80x86 processors will let you execute code in a small
}model architecture,  this is only applicable to fairly small applications
}( I know this is relatively speaking, and that 10 years ago 64k of code
}was a fairly large application).  The 680x0 family architecture ALWAYS
}fetches long word addresses (32 bits), so the most fair comparision
}is the x86 large model.

Ahem, since we're talking about a 386, the only fair comparison is to 386
small model, which is one 4 gigabyte code segment and another 4 gigabyte
segment for data/stack/heap.  Which 680x0 addresses 8 gigs at once?

}BTW, this very feature (large linear address space) on the 68k family is
}what makes it a somewhat more desirable processor to program on (no

Gee, 4 gigabytes per segment looks pretty large and linear to me....
--
UUCP: {ucbvax,harvard}!cs.cmu.edu!ralf -=- 412-268-3053 (school) -=- FAX: ask
ARPA: ralf@cs.cmu.edu  BIT: ralf%cs.cmu.edu@CMUCCVMA  FIDO: Ralf Brown 1:129/46
"How to Prove It" by Dana Angluin              Disclaimer? I claimed something?
16. proof by cosmology:
    The negation of the proposition is unimaginable or meaningless.  Popular
    for proofs of the existence of God.