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From: hamilton@uxc.cso.uiuc.edu
Newsgroups: comp.sys.ibm.pc
Subject: Re: Absolute floppy disk sector read/wr
Message-ID: <174200064@uxc.cso.uiuc.edu>
Date: Wed, 26-Aug-87 19:56:00 EDT
Article-I.D.: uxc.174200064
Posted: Wed Aug 26 19:56:00 1987
Date-Received: Sat, 29-Aug-87 10:09:02 EDT
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Nf-From: uxc.cso.uiuc.edu!hamilton    Aug 26 18:56:00 1987


/* Written  9:12 am  Aug 20, 1987 by samperi@magpie.UUCP in uxc.cso.uiuc.edu:comp.sys.ibm.pc */
/* ---------- "Absolute floppy disk sector read/wr" ---------- */
It seems that the DOS interrupt 0x25 (absolute disk sector read) can only
be used to read the first 720 sectors on a high density floppy (installed
in the high density drive of an AT). I've had to use the bios disk service
2 in order to read all 2400 sectors. Is this a DOS bug?
/* End of text from uxc.cso.uiuc.edu:comp.sys.ibm.pc */

    no, it's probably a feature.  a while back, i wanted to read some
non-dos 1.2Mb disks using the absolute disk sector routines.  i also
had a problem at 720 sectors.  however, i noticed that if i forced
dos to believe there was a 1.2Mb diskette in the drive (by DIRing a
dos disk there, then changing diskettes), it worked fine.  i suspect
that until dos sees a media type byte for 1.2Mb, it assumes any diskette
in that drive is only 360Kb.
    what i probably should have done is use the BIOS level functions.
i think there is a subfunction that allows you to tell the BIOS that
you have a 1.2Mb diskette.

	wayne hamilton
	U of Il and US Army Corps of Engineers CERL
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