Path: utzoo!attcan!uunet!cs.utexas.edu!asuvax!ncar!tank!mimsy!mojo!cyliao
From: cyliao@eng.umd.edu (Chun-Yao Liao)
Newsgroups: comp.sys.apple2
Subject: Re: Dos 3.3 Question
Message-ID: <1990Apr9.140317.6780@eng.umd.edu>
Date: 9 Apr 90 14:03:17 GMT
References: <9004082354.AA20755@apple.com>
Sender: news@eng.umd.edu (The News System)
Organization: Maryversity of Uniland, College Park
Lines: 25

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.
>
>America Online:  Klorn                     UUCP:  jefft@gnh-applesauce.cts.com

The Apple's 5.25 driver has to move the head two tracks to make one track of
data on the disk.  The reason was that the head was too wide to fit in every
track without interfere the adyacent tracks.  Therefore, Woz designed the
DOS to move twice the step motor to get one usable track.  If the head was
small enough, we can actually get 4 times of memory for each side.  Most of
today's driver can handle 1/4 track (or actually 1/2 hardware track) by the
way.  AFter all, remember that the 5.25 driver's technology is about 10 years
old, and havent changed for the compatibility.
--
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 xxxxx@xxxxx.xxx.xxx (reserved)	o RC + Apple // + Classic Music + NeXT = cyliao