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From: jwhitnel@csi.UUCP (Jerry Whitnell)
Newsgroups: comp.sys.mac
Subject: Re: SE Internal Hard Disk Size
Message-ID: <1283@csib.csi.UUCP>
Date: Fri, 6-Nov-87 14:24:12 EST
Article-I.D.: csib.1283
Posted: Fri Nov  6 14:24:12 1987
Date-Received: Mon, 9-Nov-87 05:35:40 EST
References: <549@wolf.UUCP> <189@iconsys.UUCP> <2822@batcomputer.tn.cornell.edu>
Reply-To: jwhitnel@csib.UUCP (Jerry Whitnell)
Organization: Communications Solutions Inc., San Jose, Ca
Lines: 48
Keywords: How Big ?

In article <2822@batcomputer.tn.cornell.edu> chow@tcgould.tn.cornell.edu (Christopher Chow) writes:
|||My SE with Apple's 20Mb drive formatted out to 19,019K.
||  19.07 * 1024*1024 bytes.  So.... if the routine used to put the
||  size in the title bar does it this way then you're doing pretty well.
|
|Then again, it depends on the drive.  I think my almost 1-yr old 
|Dataframe 20 has 20110 k free when you reformat it.
|
|Christopher Chow

A short lesson on disk drive construction.  A drive consists of a number
(at least one, ususally more) of platters, each of which has two sides.  These
platters are stacked on a spindle, so that both sides are accessible.  A drive
also has a read/write head for each side of the platter, except that the
top and bottom platters usually don't have any data on them, so no head.
These heads can move back and forth on the disk surface.  The disk will spin
the platters, so each head can access the whole surface of the platter.  If
you hold the head in one place and spin the disk underneath it one rotation,
the head will be able to see the circle on the platter's surface.  This circle
is called a track.  There are a fixed number of tracks on each platter,
with less then a 100 for floppies and going on up to several thousand for
large hard disks.  The number of tracks for similar size drives varies from
manufacturo to manufactuor.  Each track is further divided up into sectors
of 512 bytes each (some use 1024, some other sizes).  There is always a fixed
number of sectors per track and a fixed number of tracks per head.  So the
maximum amount of space a drive will hold is given by the formula

    heads x tracks/head x sectors/tracks x bytes/sector.

However, you may not see all of this space.  All of the good drives will
reserve some number of sectors or tracks to provide a place to map sectors
that have bad spots in them.  If the drive can't read a sector on the disk,
rather then call the whole disk bad (which is a big waste of money :-)),
it just puts the data in one of the reserved sectors and stores the information
about where the sector was moved to in a map that it keeps on the drive.  
Since the number of reserved sectors is often decided by the formatting program,
different versions of the program may give you more or less space for real
use depending on which version you use.

So for any 20 mbyte drive on the market, about the only thing you'll know
is that you'll get more then 18 mbytes and less then 22 mbytes.  



Jerry Whitnell				Lizzi Borden took an axe
Communication Solutions, Inc.		And plunged it deep into the VAX;
					Don't you envy people who
					Do all the things You want to do?