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From: stephens@latcs1.oz.au (Philip J Stephens)
Newsgroups: comp.sys.apple2
Subject: Re: Fast reading of floppies...
Message-ID: <7431@latcs1.oz.au>
Date: 15 Mar 90 05:02:47 GMT
References: <14075@fs2.NISC.SRI.COM> <14076@fs2.NISC.SRI.COM> <8613@chaph.usc.edu> <12350@smoke.BRL.MIL>
Organization: Comp Sci, La Trobe Uni, Australia
Lines: 30

In article <12350@smoke.BRL.MIL>, gwyn@smoke.BRL.MIL (Doug Gwyn) writes:
> In article <8613@chaph.usc.edu> lesatz@alcor.usc.edu (Eric Michals) writes:
>
> -Another thing that I found interesting is that Locksmith 5.0 fast disk
> -backup doesn't read the sectors in order, but rather in a strange
> -pattern. I've never taken the time to figure out why.
>
> Probably it interleaves its access to speed things up.

   To be more precise, it reads the _physical_ sectors in the order that they
appear on the disk, which translates as a different pattern of _logical_
sectors.  
   The interleaving of a standard DOS 3.3 disk runs "backwards" so to speak;
that is, the distance between sector $F and sector $E (in that order) is
greater than the distance between sector $E and $F.  Thus if you read the
logical sectors backward, you have more time to catch each one, resulting in
a _faster_ response time!  (Reading forwards causes DOS to miss the next
sector, and so it ends up waiting a full revolution for each sector). 

   P.S. A few people have requested the source code (assembly) for fast reading
of sectors, using a 5 1/4 inch drive.  I will have this source in my possession
come Monday, so if there is enough interest I may post it on
comp.apple2.binaries (or whatever it's called).  I will try and document it
enough so that the technically inclined can understand how it works.

</\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\></\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\>
<  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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