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From: bcw@rti.UUCP (Bruce Wright)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Input needed on XT Hard Disk
Summary: Controller limitation, not bus limitation
Message-ID: <3475@rti.UUCP>
Date: 19 Jan 90 17:11:59 GMT
References: <1400021@hp-ptp.HP.COM> <2590@jato.Jpl.Nasa.Gov>
Organization: Research Triangle Institute, RTP, NC
Lines: 34

In article <2590@jato.Jpl.Nasa.Gov>, kaleb@mars.jpl.nasa.gov (Kaleb Keithley) writes:
> In article <1400021@hp-ptp.HP.COM> pault@hp-ptp.HP.COM (Paul_Taira) writes:
> >      I have an IBM (real) PC-XT with a Seagate ST-225 hard disk.  I need
> >      I visited a local
> >      computer store and I was told that I could not put more than
> >      30MB on my system since it is an XT with an 8-bit bus.  Is this
> >      true
> 
> This is hogwash, I added a 60 meg Micropolis drive to my XT clone (which
> had a Western Digital eight bit controller) and it worked just fine.

Quite true - the XT bus can certainly handle disks of essentially _any_
size:  it isn't the bus that is the problem.  It's hard to believe that
someone at a computer store would make such a mistake, but I suppose it
can happen.  The problem, as Kaleb implies but does not explicitly state,
is that many older XT disk _controllers_ can't handle very many disk
sizes, and many of them assume that for relatively low-end machines you
wouldn't _want_ very big disks anyway, so the controller doesn't have the 
support for big disks.

If you give it even a minimal amount of thought, the absurdity of the
idea of the XT's bus not being able to handle big disk drives is obvious -
the only time the disk size makes any difference is in specifying the
disk address (cyl, track, sector) which is usually done in I/O space, not
in memory space;  the memory addressing limitations of the XT don't even
enter into the picture.  There _are_ some problems taking advantage of
_high performance_ disk drives on the XT, where the width/speed of the
bus would be a handicap to using the drive as effectively as one would
like (you may not be able to use that nice fast drive with a 1:1 inter-
leave factor, you might have to settle for 1:3 or 1:5 which for a fast 
drive might not be great but which would still not be all that bad),
but the drive would still _work_ if you got a controller to support it.

						Bruce C. Wright