From: lasner@watsun.cc.columbia.edu (Charles Lasner)
Subject: teledisk observations.
Reply-To: lasner@watsun.cc.columbia.edu (Charles Lasner)
Organization: Columbia University
Date: Tue, 18 Aug 1992 01:59:03 GMT

Teledisk apparently does corrupt certain disk images using the so-called
"advanced" compression, but there is a companion program TDCHECK to use
to ensure the file is viable.

I can't make an RX50 corrupt however, so using this program should make the
program viable to use for our purposes, as long as TDCHECK is used to ensure
the validity of the image file.

There is another problem with TELEDISK that doesn't apply to RX50: FDFORMAT
diskettes for high-capacity PC usage.  The standard high-density 3.5" diskette
yields 80 tracks x 2 sides x 18 sectors/side or exactly 1.44 MBytes.  By
using a 2:1 interleave, it is legitimate to cut the trailing gap down because
the "cool-down" area becomes the sector you are skipping over, thus allowing
21 sectors, and also you can use 82 tracks.  This gives a total capacity of
1.72 MBytes.

TELEDISK attempts to copy this disk into an image file, and generally does
succeed wrt the CRC problem reported, but it creates a disk whose contents
is entirely low-level formatted correctly, and entirely *wrong*!  It has
been confirmed that the formatting is correct though, so it would seem to
be a bug as opposed to a limitation.  Note that for this particular format,
it is possible to use alternate means to accomplish what TELEDISK does:

From FDFORMAT, there is a companion utility called WIMAGE.

You can create an image of any standard DOS disk, including all of the
FDFORMAT variations, such as the 1.72 Meg diskette.  Usage is:

WIMAGE D: file

to create an image file, or

WIMAGE file D:

to create a diskette from a file.

The diskette must already be formatted, and the file isn't compressed.  This
allows us to use any compression method we choose, which is more versatile
than the two options offered by TELEDISK.  But the diskette has to be an
MS-DOS one, and only the kind recognized by FDFORMAT.  For example, FDFORMAT
doesn't recognize Rainbow/DECmate MS-DOS since it uses an alternate when 
dealing with RX50 80 track 10 sector format.  This alternate format isn't
recognized on the DECmate or Rainbow, and on a PC is accessed directly and
without a driver.  And of course, non-DOS disks are out of the question.

DR-DOS 6.0 offers another method:

DISKCOPY and DISKCOMP will recognize any MS-DOS disk that is accessible, 
including MS-DOS RX50 diskettes accessed by RAINDOS (curiously, not by
RX50DRVR).  Contrast this with MS-DOS that only recognizes standard disk
sizes.

Thus it is possible to do pretty much the same thing here:

DISKCOPY B: file

or 

DISKCOPY file B:

The overall operation is similar to WIMAGE.  The files created are slightly
longer because of some internal overhead, and aren't compatible with WIMAGE.

DISKCOMP is compatible with these files from DISKCOPY.  Again, the restriction
is that the files have to be valid MS-DOS files, which is marginally less
restrictive than WIMAGE, since compatible MS-DOS RX50 is allowed.  Output 
target diskettes must be correctly preformatted, as with WIMAGE, but are
not checked for pre-existing DOS information.  Thus a frill feature is
achieved: you can copy an RX50 MS-DOS diskette to another RX50 diskette
(perhaps indirectly through an intermediate file) where the target RX50
diskette was formerly used for non-DOS purposes.  (WIMAGE complains if
the high-level format of the target diskette is different from the high-level
format of the file.)

Thus TELEDISK is still needed for dealing with the myriad formats of RX50
that aren't DECmate/Rainbow MS-DOS high-level format.  This includes:

All OS/278 variants including most DECmate install diskettes, test diskettes,
CBI diskettes, etc., as well as OS/278 itself (DM-101,DM-111) and other
DECUS distributed diskettes.

CP/M-80 diskettes on DECmate (PRO?)

CP/M-86/80 diskettes on Rainbow

DECmate Master Menu volume backup format

P?S/8 diskettes (two varieties)

COS-310 diskettes

WPS-8 diskettes

WPS-8 system diskettes

WPS-8 utility diskettes

WPS-8 DECspell diskettes

WPS-8 autopatch diskettes

VENIX diskettes

FILES-11 (RMS) diskettes

Anyone want to add to this list?

Moreover, some of these diskettes require non-standard stagger and interleave
factors applied, in zones in some cases.  Teledisk does support these
format variations.  Thus I don't see a replacement for TELEDISK being available
that serves all of this.

cjl




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