Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!deimos!ux1.cso.uiuc.edu!cs122aw
From: cs122aw@ux1.cso.uiuc.edu (Scott Alfter)
Newsgroups: comp.sys.apple2
Subject: Re: Fast reading of floppies...
Message-ID: <1990Mar14.234750.27756@ux1.cso.uiuc.edu>
Date: 14 Mar 90 23:47:50 GMT
References: <14075@fs2.NISC.SRI.COM> <14076@fs2.NISC.SRI.COM> <8613@chaph.usc.edu>
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Organization: University of Illinois at Urbana
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In article <8613@chaph.usc.edu> lesatz@alcor.usc.edu (Eric Michals) writes:
>Maybe it is just my drive, but I've also noticed that reading from
>sector $F to sector $0 is quicker than from $0 to $F.
>
>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.

What you're talking about is interleave.  Yes, that property of hard disks that
GS owners are constantly playing with is a property of every random-access
mass-storage medium ever developed.  The computer will read in a bunch of 
data, encoded 6-and-2 on the disk, and take some time to decode it into the
familiar 256 bytes per sector.  (342 disk bytes are required to store one
sector; the reasons are beyond the scope of this post.)  In that time, another
sector is whizzing by the drive head, so if it was the next sector you needed,
the disk will have to make a full spin before the computer can catch it again.
Interleaving lets you scatter sectors on a disk so that when the computer is
done processing one sector, the sector coming up on the disk is the next one
the computer will read; the one that went under the head while the computer
was chewing the numbers from the last sector is not what DOS is looking for, so
the disk doesn't have to make a full spin to get the next sector.  Locksmith is
set up to take advantage of interleave to read sectors in the most efficient
manner possible.

Scott Alfter-------------------------------------------------------------------
Internet: cs122aw@ux1.cso.uiuc.edu    _/_  Apple II: the power to be your best!
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