R: Floppy disk: one drive per face

Enrico email.it enrico.lazzerini at email.it
Thu Nov 5 16:42:03 CST 2020

About 5,25" diskettes it seems that its geometry could be based on 128byte
for sector so using FM recording way and 40 tracks for side it could have 18
sectors for track. If I try to format in this way a disk with 22disk program
I instead get 16 sectors for track. Having doubt on this we gone back to
look better on 8" diskettes to look deep about the way on which the b side
could be read. We can image that on 1980 they written before a way to record
just on one side of the diskettes and then they added double face diskettes
handling them as two separate drives (leaving the same sel 0 pin but
changing the side select pin to 1), so: why would have they rewrite all
instead of to replicate the same what already way made well on the first
side diskettes?

-----Messaggio originale-----
Da: cctalk [mailto:cctalk-bounces at classiccmp.org] Per conto di Fred Cisin
via cctalk
Inviato: giovedì 5 novembre 2020 22:50
A: General Discussion: On-Topic and Off-Topic Posts
Oggetto: Re: Floppy disk: one drive per face

On Thu, 5 Nov 2020, Enrico email.it via cctalk wrote:
> We are trying to read some floppy disk used on a 1981 Italian Z80 machine
> with CP / M.
> The problem is that the firmware selects face A of the floppy as DRIVE A
> face B of the same floppy as Drive B.
> We saved an image with IMD and we know that the geometry of face A is IBM
> 3740 26x128x77 tracks.
> By inserting this data into the 22DISK definition file we can successfully
> extract the CP / M files from A side.
> How should we configure 22disk to be able to do the same thing on the B
> of the same floppy disk?
> Inserting Side 2 in the definition table is not enough ....

An irrelevant data point, . . .
when double sided floppy drives came out for the PC/5150, there were some 
patches informally distributed (ISTR mimeograpahed), for doing that with 
PC-DOS 1.00!  But, very soon thereafter, PC-DOS 1.10/MS-DOS 1.25 came out 
that supported a full double sided format.

Is this an 8" disk?  SSSD?   (128 x 26 x 77)
(As you are presumably already aware, many PC FDCs have problems with FM 
and/or 128 bytes per sector)

A really stupid HARDWARE way to do it would be to swap the wires for the 
heads on the drive!  Many drives have convenient plugs that would make 
that easy.
Another stupid hardware way to do it would be to jumper side select, to 
falsely select second side.

Chuck has provided a straight-forward solution.  There were several 
different ways that disk formats were expanded to double sided.
Some went up side A, and then DOWN side B
Some made use of the faster head switching than track stepping, and did a 
track on one side, then a track on the other side.  (which is the "modern" 
standard, for MS-DOS, etc.)

SOME went up side A, and then went back to track 0 and then up side B.
With one of those formats, you could simply tell it that the "reserved 
tracks" occupied the entire first side.  (77 reserved tracks from your 

XenoCopy does not have such a format supported.  I am not going to be 
adding it at this time.

Uniform does not support it.

22Disk apparently has easily modified parameter tables, so should work 
just fine.  Thank Chuck!

Grumpy Ol' Fred     		cisin at xenosoft.com
XenoSoft                        http://www.xenosoft.com

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