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From: uhclem@trsvax
Newsgroups: net.micro.trs-80
Subject: Re: Orphaned Response
Message-ID: <70700063@trsvax>
Date: Wed, 21-May-86 12:07:00 EDT
Article-I.D.: trsvax.70700063
Posted: Wed May 21 12:07:00 1986
Date-Received: Sun, 25-May-86 17:18:51 EDT
References: <1184@ncoast.UUCP>
Lines: 45
Nf-ID: #R:ncoast.UUCP:1184:trsvax:70700063:000:2106
Nf-From: trsvax!uhclem    May 21 11:07:00 1986


<This line is invincible!>

Regarding:
>It [diskutil] does
>not ask the precomp time for the drive, nor the cylinder at which
>precomp is changed.  This means that most any drive can be formatted
>but that some drives may have problems that accumulate over time
>as the mismatch between the Tandy precomp and the precomp the drive
>requires begins to cause read/write errors.  This has not been a

If you are worried about precomp mismatches, simply modify the PVH
which diskutil writes to the beginning of each hard disk after you
have formatted the disk.

The first block on /dev/hdbtx will be the PVH.  You can use the patch
utility to change the precomp value to whatever your particular drive
requires.  XENIX picks up this value each time the drive is opened
and uses it for that drive.

The offset for PRECOMP is 0x0e == PVprecomp
This word value is stored in non-word-swapped order.
Also keep in mind that the WD controller takes that value, multiplies
it by 4 and that will be the starting cylinder for precomp, so you
only have a +/-4 cylinder resolution on the starting point.
WD told me in the past to turn on precomp earlier than needed, or to leave
precomp on All the time as their data sep. can cope with that.

Diskutil programs one-half the drive cylinder count (divided by 4), which is
correct or very close to all the drives that RS sells which care about the
programmed value. (Certain models ignore the precomp value and use their head
positioning register to determine when precomp is on.  A lot of the servo
models are like this, so you might check with the drive maker of yours.)

By the way, it really isn't a SCSI bus.... The Bernoulli box uses that
and the hardware and software schemes are totally different.

<This information is provided by an individual and is not nor should be
 construed  as  being  provided  by  Radio  Shack or Tandy Corp.  Radio
 Shack/Tandy Corp has no obligation to support the information provided
 in  any way.  Besides, the market for those 8meg Shugarts may return!>
						
						"Thank you, Uh Clem."
						Frank Durda IV
						@ <trsvax!uhclem>