Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cwjcc!gatech!udel!rochester!pt.cs.cmu.edu!b.gp.cs.cmu.edu!Ralf.Brown@B.GP.CS.CMU.EDU
From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.sys.ibm.pc
Subject: Re: Question about diskette densities
Message-ID: <24a77177@ralf>
Date: 27 Jun 89 12:22:15 GMT
Sender: ralf@b.gp.cs.cmu.edu
Organization: Carnegie Mellon University School of Computer Science
Lines: 49
In-Reply-To: <8465@killer.DALLAS.TX.US>

In article <8465@killer.DALLAS.TX.US>, chasm@killer.DALLAS.TX.US (Charles Marslett) writes:
}I have had quite a bit of experience mailing diskettes, moving code from
}one machine to another, etc, with 9 and 10 sector/track diskettes.  And
}the 9 sector/track format works going from ATs to XTs, from Fujitsu drives
}to Teac to Cannon to ...  On the other hand, 10 sector/track diskettes are
}readable at the destination (usually), but the first time the dummy who
}received the diskette tries to write anything to it - BOOM goes the FAT,
}and nothing is accessable on the disk ever again!
}
}In article <7710@cbnews.ATT.COM>, cbema!las@cbnews.ATT.COM (cbema!las) writes:
}> In order to make room for 10 sectors per track on 5 1/4 inch floppy
}> disks, you must reduce the gap between sectors to a value less than
}> the "standard" originally defined by IBM (let's see... I think they
}
}The original gap was 42 bytes (out of about 330) for 8" media, for 5 1/4"
}media it was increased to 48 bytes (the drive power supplies and moters
}were not so reliable), and to add a tenth sector it has to be set to
}around 15 or so.  Since the floppy data seperator requires 10 bytes or

The standard gap for 8- and 9-sector formats is 42 bytes.  I believe JFORMAT
uses a gap of about 25 bytes (it's been a few years since I had my hands on
a copy) for 10-sector disks.

}IBM did not change anything when they went from 8 to 9 sectors/track,
}since the difference was a performance and simplicity loss on one side
}in exchange for a capacity gain on the other (old drives stepped very
}[...]
}
}It [10-sector format] had no effect on the DOS mainstream, mostly because
}it would never have
}been accepted by software vendors, so anyone who used it would have to
}support two formats (read, more work, no more pay, never get done ;^).
}No major vendor thought it worth the effort.

Then there was the Eagle PC, whose version of MSDOS 1.25 supported 400K disks
using 5 1024-byte sectors per track (and a 79-byte gap).  IO.SYS used a nice
table-driven approach to disk formats, which let me patch the (for me unused)
entry for 800K disks (like 400K but 80 tracks) to read and write 360K disks
without subdirectories.  That particular version of DOS what ahead of its time
in other ways, too, including support for 10M and 20M hard disks and a
SYSINFO.SYS file specifying the default printer, fast/slow scroll,
block/underline cursor, etc.

--
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