Using 3.5" HD drives on CP/M systems

From: Herb Johnson <>
Date: Tue Feb 1 13:10:26 2005

"Randy McLaughlin" writes [in exerpts,, my text in brackets]:

>I have tried to get a quorum [about a standard use of 3.5-inch floppy
>drives on old CP/M systems] on comp.os.cpm but there are only a few
>with any opinions.
>We all agree that 3.5" drives are a viable way to go. Many prefer to
>never change from 8" drives. I am running the SuperIO
(, >it is a new Imsai card and uses a PC controller.
>I have a variety of CP/M systems with a variety of storage mediums.
The >point is for those of us that use 3.5" HD drives should try and
come to >a consensus for a "standardized" format. Questions such as
sector size >and numbering come to mind.
>With more PC's dropping 3.5" drives does that mean there will be a
>limit to new drives all together?
>[mention of USB drives and Flash drive options snipped]

I think Randy is looking for a CONSENSUS, not a "quorum", but in any
event he is looking for an acceptable standard. He did not get a lot of
responses because 1) it's only been a few days, and 2) it's a
technically tough subject, and 3) it's an OLD subject over the years.

Also: there is some irony in Randy's suggestion that resistance to 3.5"
drive use amounts to "never change from 8-inch". People who use 8080
systems at 1, 2, 4MHz may be reluctant to "change" those systems to use
USB or even 3.5" diskettes - go figure!

But a standard can't be enforced universally, because old hardware has
unique problems. Also, it's not likely sufficient software will be
available for every system's use, particularly if their owners are not
programmers who can modify available code. With an oddball "standard"
for your old computer's floppies, instead of the original standards for
that brand and model, you lose compatibility not only with OTHER systems
of the same brand and model, you lose the ability of yet other systems
which can still read the old disks, because THOSE computers don't
support that new "standard"! And when your old computer dies - or YOU
die - how do your old diskettes get read?

If Uncle Fred's old Kaypro now writes "Standard 2K" disks only, what
happens when Fred dies and his nephews and nieces want to recover his
poetry files? "I have these Kaypro disks, but Fred's Kaypro was
discarded. But Barney's Kaypro can't read them, Wilma's IBM PC "readall"
program can't read them in Kaypro format - what do I do"? Randy is right
in part - you have to have a UNIVERSAL, well-supported standard, across
MANY MANY systems, for a "standard" to work well. Otherwise you risk
"write only" diskettes.

The other problem in general is hardware - CPU, floppy controller,
drive, media.

The fundamental hardware problem is slow 8-bit processors, not to
mention the oldest floppy diskette controllers. Early 8080 systems ran
at only 1MHz or 2MHz; they can at best barely keep up with single
density (FM) floppy disk formats, due to limits at which that processor
can keep up with the data from a floppy disk controller chip. A VERY
THOROUGH presentation of that point, with source code, is by Bruce JOnes
and is mostly available on my Web page:

A list of floppy disk drive and media technical specs is on another page
of mine:

Randy suggested in his comp.os.cpm posts that it's basically OK to write
in single density FM format on 3.5" high-density (HD) media, as he says
the "flux changes in FM and MFM" are at the same rate. For the oldest
computers, FM is the ONLY format option you may have. I did not respond
to Randy's post. But I think the problem of that scheme for use at 3.5"
HD will be two-fold. First, the actual data rates of old single density
FM 8-inch FDC's are much slower than standard data rates for HD 3.5"
media; the same applies for the read/write electronics of the DRIVE.

Second, no "modern" computer which uses 3.5" 1.4M drives is likely
CAPABLE of reading an FM encoded (single density) data stream, except by
accident. This calls to mind a classic problem on IBM compatibles,
namely reading old 5.25" 360K SINGLE density diskettes (like Osborne
diskettes). The FDC chip and/or decoder does not "expect" an FM encoded
data stream.

Frankly, I find this kind of discussion VERY CONFUSING, as there are a
number of design specifications in play. Note all the technical issues
above: media, data rates, read/write electronics - not to mention how
drives are switched, or switch "themselves" between these various
schemes. As such discussions have occured many times before in
comp.os.cpm, I decided to compile the best posts and primary-source
technical data on my own Web site, and to refer others to it when
appropriate. (My site draws no conclusions, it just posts specs, code,
and other's technical discussions.)

In my opinion, the "point person" on issues of disk and drive is
Amardeep S Chana. He has posted for years on that subject and his
technical knowledge and experience are apparent in his many posts.
Bruce Jones has "walked the walk" and written code and documents how it
works, which he shares freely and which others have put to use. Both
have content on my site, with their permission, via the links above.

One personal note. This took a few hours to write. It takes time to make
a good, technical "argument". It took time for me to complile all the
info on my site, and time for THOSE people who wrote it to do their
work. It does not take long to propose a popular idea, and it's fun;
it's less popular (and more work) to say "here's why it may not happen".

Herb Johnson


Herbert R. Johnson, voice 609-771-1503, New Jersey USA
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Received on Tue Feb 01 2005 - 13:10:26 GMT

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