Re: Making disk images of NorthStar CP/M disks with Catweasel

Dave Dunfield wrote:

(snip)

However, here is where things get interesting. I was looking at the
"bad" sectors and noticed it was eight sectors all in a neat little
row, five on one track and three on another directly adjacent. Sort
of like a file would be if it were written to a disk. :-) So I tell
the Catweasel and cw2dmk to do a "raw bit dump" of those two tracks
and take a closer look at the "bad" sectors. The raw data look
suspiciously like "good" sectors but the MFM decoder can't seem to
make sense of them... you guessed it! The "bad" sectors are really
FM single density sectors from files randomly mixed in. ARGH! Mixed
density disks are the bane of disk imaging!

I remember them from tapes in OS/360 days, too. If you add a new
file to (the end of) a tape, the system reads the tape up to the
point where the new file is to be added, and then starts writing
at the specified density. If the drive can read the existing density
it will properly read up to that point, and then write as it was
asked to do. Such a files are only readable on drives that support
both densities.

Yes, NorthStar DOS supports saving individual files in single or
double density, and it's possible to mix density "randomly" on
a single disk.

I thought I remembered some system where the boot tracks are written
as SD, the rest as DD.

In practice I've not encountered this except for disks that I made
to see that it does in fact work. You may not have noticed, or
forgotten it, but this statement appears in my NST documentation:

The DD format that DEC uses for the RX02 drive uses FM headers on
MFM data blocks. If you write a 256 byte sector, it should then take
up the same amount of space as a 128 byte FM sector.

-- glen

.

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