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From: cs225af@ux1.cso.uiuc.edu
Newsgroups: comp.sys.apple
Subject: Re: Dos 3.3 Question
Message-ID: <15800114@ux1.cso.uiuc.edu>
Date: 9 Apr 90 09:38:48 GMT
References: <9004082354.AA20755@apple.com>
Lines: 22
Nf-ID: #R:<9004082354.AA20755@apple.com>:-30:ux1.cso.uiuc.edu:15800114:000:1187
Nf-From: ux1.cso.uiuc.edu!cs225af    Apr  8 20:59:00 1990



> 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.

The stepper motor in the Disk II and similar drive mechanisms is capable of
moving on QUARTER-tracks.  But the read/write head is not sensitive enough to
be able to use the fractional tracks AND the normal tracks... there is too 
much crossover between them.

Many copy-protection schemes, though, would use half tracks by, for example,
writing data to track 16, track 17.5, and track 19, eliminating the crossover
from adjacent half-tracks but effectively hiding the data on the half-track
from less sophisticated copy programs....

--rubio  (rubio-1@uiuc.edu)