Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!think!husc6!m2c!wpi!dseah
From: dseah@wpi.wpi.edu (David I Seah)
Newsgroups: comp.sys.apple2
Subject: Re: Dos 3.3 Question
Message-ID: <11204@wpi.wpi.edu>
Date: 9 Apr 90 04:20:50 GMT
References: <9004082354.AA20755@apple.com>
Reply-To: dseah@wpi.wpi.edu (David I Seah)
Organization: Worcester Polytechnic Institute, Worcester ,MA
Lines: 23

In article <9004082354.AA20755@apple.com> jefft@gnh-applesauce.cts.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? To read a full
>track, SEEKABS must be called in a loop twice, essentially seeking a double
>half track to yield a full track, which seems to be wasteful, unless The Woz
>originally had plans of formatting or using half tracks and then abandoned it.
>I wanted to know if anyone knew why this was done in this way.

Yep!  SEEKABS is called twice to step for "half tracks".  The reason
why DOS 3.3 doesn't use half tracks is because the read/write head
is too wide...attempt to write on an adjacent half track will destroy
the original track.  I'm not sure if its possible to actually step two
half tracks in one go...my understanding was that the stepper motor
was such that you couldn't do anything else.  But then again, there
were quarter-track protections...coupled with other relatively simple
synchronized and spiral-tracking routines would break just about any
automated bit copier, hehehe.
-- 
Dave Seah | O M N I D Y N E  S Y S T E M S - M |  "I sacrifice this cat to
          |   User Friendly Killing Machines   |     Cognitive Science!" - MEK
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I-net: dseah@wpi.wpi.edu - America Online: AFC DaveS (Apple II Art & Graphics)