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From: stephens@latcs1.oz.au (Philip J Stephens)
Newsgroups: comp.sys.apple2
Subject: Re: Dos 3.3 Question
Message-ID: <7632@latcs1.oz.au>
Date: 9 Apr 90 01:44:38 GMT
References: <9004082354.AA20755@apple.com>
Organization: Comp Sci, La Trobe Uni, Australia
Lines: 30

In article <9004082354.AA20755@apple.com>, Jeff Tarr Jr. writes:
> 
> Okay, I was dissassembling the SEEKABS routine in the DOS 3.3 code that seeks
> the read/write head and it appears that DOS 3.3 has the capability of seeking
> and writing to half tracks as well as full tracks. I was wondering what the
> reason for this was, and does this fact slow down disk access? 

  The disk drive has 80 tracks (which you've called "half tracks"), devided
into groups of 4.  A four-phase stepper motor moves the arm from track to
track in either direction.
  Since the read/write head is too big, you can only use every second arm
position as a track.  However, you can't just skip over every second track
position because of the way the stepper motor works, which is why the routine
is called twice.  Basically, to move the arm from one position to the next,
you must turn on the destination phase, delay for a while until the motor
gains enough speed (or something to that effect), and then turn off the source
phase.  I'm pretty sure you are not allowed to skip over a phase - or at least,
the head doesn't move properly if you try it.  (I may be wrong, of course.)
  Thus, I don't think there is a faster way of moving the head, except for
playing around with the current phase delays (which are probably optimal
already).
  [Just as an aside, the image of a track tends to "leak" over onto the half-
track positions, which is taken advantage of by Locksmith's disk recover
function in order to restore trashed disks.] 

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<  Philip J. Stephens                ><   "Many views yield the truth."        >
<  Hons. student, Computer Science   ><   "Therefore, be not alone."           >
<  La Trobe University, Melbourne    ><          - Prime Song of the viggies   >
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