[Simh] 8" Floppy disk image getting HALT error

[Ethan Dicks]

On Mon, Jul 10, 2017 at 4:36 PM, Clem Cole <clemc at ccc.com> wrote:
> Paul - offered great info.   First are your sure these came from DEC
> systems?  8" floppies were originally developed (by IBM to load microcode on
> the 3330 disks) years before DEC started to use them.

Yep, but if they are RX02 format (single-density headers,
double-density data), then they can only be from a DEC system.

>   IIRC DEC only used them on PDP-8's and 11s.

Not strictly true.

> The Vaxen and PDP-10 actually connected to a PDP-11 front end that actually controlled them.

Many VAXen, 11/780s and similar, did have a console RX01 attached via
a console PDP-11, but you were free to stuff an RX11 into a Unibus
slot and read floppies directly from VMS, for example.  It was
uncommon, I'll grant you, but it was done.  There was a pair of
11/750s at OSU, Scarlet and Grey, used in the Engineering Graphics
140-series in the mid-1980s that used student-accessible RX01 drives
for saving their work.  Uncommon, but possible.

> Second, as Paul said, trying to boot from them, unless you know what you are
> doing is likely to cause a certain amount of madness, be a tad random.

Yep.

> when dealing with floppies, you have to remember two important things....
> format (hard or soft) and interleave.    Early [8" in particular] used hard
> formatting.   I don't think DEC's version of the 8" ever did [ Paul is more
> likely to remember that type of factoid].

Nope.  They did not.

>> I can’t go back to the donors and ask what machines these 8” floppies were
>> used with, so I’m not sure how to begin troubleshooting.

I would start by attempting to read them as RX01.  That low-level
format is identical to IBM 3740 format and covers a wide range of
single-density machines.  If it really reads successfully as a 500KB
RX02, then it's a DEC RX02 and likely nothing else.

Grubbing around the resulting image file with hex or octal dumps,
especially on the first track, will likely reveal what sort of system
it's from.  If the source is DEC, it's most likely to be PDP-8 or
PDP-11, but I happen to have a few diskettes from a 3rd-party disk
system that, while it was on either a PDP-11 or PDP-8 (the labels in
my case are helpful), they are *not* RX01 or RX02 compatible.  DSD
drives were semi-common for some cost-conscious DEC users, and while a
Kryoflux can certainly be told how to read them, it won't be with
either an RX01 or RX02 description file.

Again, if you get meaningful sectors out of an RX01-formatted read,
I'd start there.  The IBM 3740 format was widely used.  If that turns
up garbage, then it's a harder hunt for a match.

-ethan