Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!apple!bionet!ig!uwmcsd1!marque!uunet!seismo!sundc!pitstop!sun!imagen!atari!apratt
From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: How accurate is that show info. thing?
Message-ID: <1183@atari.UUCP>
Date: 6 Oct 88 19:41:38 GMT
References: <881004014748.973404@PCO-MULTICS.HBI.HONEYWELL.COM> <1545@nunki.usc.edu>
Reply-To: apratt@atari.UUCP (Allan Pratt)
Organization: Atari (US) Corporation, Sunnyvale, California
Lines: 29

It's worse than you think:  if you create a file and write 10 bytes to
it, 1014 bytes are being "wasted" -- files are allocated in "clusters"
of 1K each.  This is called "internal fragmentation" and is the kind
of overhead you have to deal with.  The alternative is to allocate files
in bytes, which yields zero internal fragmentation but is REALLY SLOW
and yields LOTS of external fragmentation.  External fragmentation
is the space used to keep track of the data: directories (including
the root directory) and FATs.

Show Info on the desktop adds up the number of data bytes used in each
file; that's the "bytes used" field.  Then it uses the OS Dfree call to
determine bytes available.  You will notice that the bytes available is
always an even multiple of 1K.  The difference between (bytes used +
bytes available) and (bytes available on an empty disk) is the internal
fragmentation plus subdirectory space.  (Root directory and FAT space
is not counted in the "bytes available" field for an empty disk).

The Dfree call returns several numbers: the number of clusters on the
disk to begin with, the number of clusters currently available, the
number of disk sectors per cluster, and the size of a disk sector in
bytes. 

Note that a freshly-formatted disk will show two clusters already used. 
This is a bug in GEMDOS.  It can be fixed, but we decided not to,
because it creates problems with new versus old ROMs. 

============================================
Opinions expressed above do not necessarily	-- Allan Pratt, Atari Corp.
reflect those of Atari Corp. or anyone else.	  ...ames!atari!apratt