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1989-04-14
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AT720 - BIOS PATCH FOR 720K DISKETTES IN 1.2M DRIVES
Chuck Guzis
Sydex
153 North Murphy Ave.
Sunnyvale, CA 94086
(408) 739-4866 (voice)
(408) 738-2860 (BBS)
We've had several users of our Con>Format and FORMATQM utilities call in
inquiring about the 720K diskette format on 1.2 Mbyte high-density 5.25"
drives.
Briefly, this format is identical to the 3.5" 720K format, placed on a
5.25" diskette. This mode does not require high-density diskettes, nor
does it sacrifice reliability for a doubling of the available diskette
space. Normally, when a 360K diskette is formatted in a high-density
drive, every other track of the diskette is formatted, with the alternate
tracks left blank. This is because the high-density drive crams twice
as many tracks (80) into the same space as the 40 tracks of the double-
density 360K drive. The 720K format merely uses every track available.
Unfortunately, some BIOS implementations for the AT clones make the
assumption that if a 5.25" diskette isn't high-density, then the diskette
must be 360K. This results in "Sector not found" errors. Most Phoenix
and IBM BIOS implementations do not have this problem, but some of the
less-common versions, such as the AMI BIOS do.
AT720 is a VERY small resident which fixes this problem in those un-
cooperative BIOS sets. Just put a command of the form "AT720" in your
autoexec.bat file, and the problem will be taken care of.
We don't ask anything for this program--in order to make use of the 720K
format, you'll have to register one of our formatters--and we'll get your
money there.