Path: utzoo!attcan!uunet!decwrl!wuarchive!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!dali.cs.montana.edu!milton!uw-beaver!entropy!dataio!shiloh!fnx!del
From: del@fnx.UUCP (Dag Erik Lindberg)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Making a floppy drive ignore high density hole
Message-ID: <598@fnx.UUCP>
Date: 25 Jul 90 23:39:23 GMT
References: <11547@ingr.com> <1990Jul19.021714.10040@world.std.com> <803@digi.lonestar.org> <1184@gold.GVG.TEK.COM>
Reply-To: del@fnx.UUCP (Dag Erik Lindberg)
Organization: I/Ovations    Kirkland, WA
Lines: 32

In article <1184@gold.GVG.TEK.COM> grege@gold.GVG.TEK.COM (Greg Ebert) writes:
>
>The difference between 720K and 1.44M formatted diskettes is single vs. double
>density, and not narrower tracks as in 1.2M floppy drives. All 5.25" drives
>are asumed to be double density.
>

Bzzzzzt.  Wrongo.  No existing standard floppy controller for the PC family
(after-market-special-whizzbang-read-everything-in-the-world controllers
excepted) can read or write single density.  It is true that recording
frequency and encoding method change between 720k and 1.44M, but 720k is
definitely *not* "single density".  It is in fact the exact same format
used on 360k DSDD (that's "Double-Sided-Double-Density". Ever wonder why
you see that on standard 3.5" disks?), which is why even the original
IBM PC can be coaxed into reading/writing 3.5" disks.

>My MSDOS technical information regarding FATs, etc is a bit outdated, but it
>does describe single and double-density 8" diskette support. Basically, an
>attempt is made to read the disk in single density. If a read-error occurs,
>an attempt will be made using double density. I conclude that it is a BIOS
>constraint whether your 3.5" drive is 720K or 1.44M. Any BIOS gurus out there
>wish to comment ?

There is indeed a BIOS constraint.  But also a hardware constraint.  Mainly,
if your machine has a FD controller that can handle 1.2Meg 5.25" disks, then
you can handle 1.44M 3.5" disks, since the recording format is essentially
the same!  Yes, of course there are some minor differences, stress *minor*.


-- 
del AKA Erik Lindberg                             uunet!pilchuck!fnx!del
                          Who is John Galt?