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From: brewster@watdcsu.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why 4.77MHz on clones?
Message-ID: <2977@watdcsu.UUCP>
Date: Mon, 2-Feb-87 23:49:36 EST
Article-I.D.: watdcsu.2977
Posted: Mon Feb  2 23:49:36 1987
Date-Received: Tue, 3-Feb-87 22:04:28 EST
References: <1292@ho95e.ATT.COM> <5426@amdahl.UUCP> <1326@whuts.UUCP> <1696@hoptoad.uucp>
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Organization: U of Waterloo, Ontario
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> > A lot of PC clones offer two speeds, 4.77 and 7.x or 8.0 MHz, and claim
> > the 4.77 is for compatibility.  What is the compatibility problem?
> > Add-on hardware?  Stupid copy-protection?  Do I need to care?

> Side_note: Ever wonder why 4.77 was picked?   The 8088 bus was designed 
>around the Multi-bus specs.  As I remember 4.77 or some mutilple was use
>on the Multi-bus for timeing.
>Tim Pozar

    Compatibility problem : early PC software used software timing loops,
    which will fail when you change the speed that code executes at.
    
    Why 4.77 MHz ?  See IBM Tech Ref 1-4

    The microprocessor operates at 4.77 MHz.  This frequency is derived
    from a 14.31818 MHz crystal, the frequency of which is divided by 3
    for the microprocessor clock, and divided by 4 to obtain the 3.58
    MHz color-burst signal required for color televisions (also used
    by colour televisions in Canada).

    Result : 4.77 MHz too save cost of 1 crystal.

    Copied without permission but in the spirit of page v of the preface :
    "the information in this manual is for reference".

    The original PC was put out at a time when running the processor
    at 7.15 MHz, if even possible, would have been far too risky a
    move for a conservative company like IBM.  I believe that it was
    only last year that you could start to buy a 6 MHz AT from IBM,
    where clone companies talk of selling 16 MHz AT's, and can 
    probably deliver reliable 10 MHz product.
                                                   
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