8" floppy project

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue Aug 10 16:41:57 2004

> >AFAIK, Teledisk only handles 'standard' disk controllers. It wouldn't
> >work with a Catweasel card, for example. Which immediately limits the
> >disks it can be used with.
> I'm no expert on disk but IIRC there's NO software for the Catweasel
> except what you write yourself so why does the make Teledisk any less
> appealing?

For the very simple reason that the Teledisk archive format is
undocumented, which means the only thing that can read/write it reliably
is Teledisk itself. Which means that you can't use the Teledisk format as
a universal archive format (which is what we are discussing here), since
you couldn't, say, write a program that used the Catweasel to read an
Apple ][ disk and store the result in a Teledisk-structured file.

Let me give a real example of when this could be useful. Suppose you have
a PDP11 running RT11 with 8" floppies. Now, DEC made 2 types of 8" floppy
unit, the RX01 which used an industry-stnadard FM (single-density) format
and the RX02 which used a very strange DEC-specific format.

To make life more interesting, the classic-PERQs have a floppy format
called 'Interchange Format' which is very similar to the RT11 filesystem.
Similar enough that a PERQ can read/write a DEC RT11 disk, and a PDP11
running RT11 will read/write(with one minor problem) a PERQ interchange
disk (the difference, for those who are interested, is that the PERQ
interchange disk has one extra word in the directory entry of each file
which is used to provide a true file length value, not just the number of
;blocks used). The PERQ hardware uses an NEC 765 controller (same as a
PC) with a data separator capable of working in FM (single density) or
MFM (double density) operation. The single density is compartible wiht
the DEC RX01, the double density is, of course, incompatible with the
RX02. But any PC controller capable of high density operation can read
the double denisty PERQ disks, and PC controllers capable of single
density operation can read the single diensity PERQ disks and DEC RX01s.

Now remember that the file system is essentially the same on all these

It is possble to design an interface to link a DEC RX02 up to a PC (the
interfave to the PDP11 bus is quite simple, doing it to ISA wouldn't be
much worse). In which case you could read these strange DEC
not-quite-double-density disks on a PC.

But what you couldn't than do is create a Teeldisk archive of an RX02
becuase the format of that archive isn't documented.

And suppose you want to do more with the disk images than just write them
back to disk. Maybe use them with an emulator on the PC. Maybe extract
files from them. Since the format of the Teledisk archive isn't
documented you probably couldn't do this. And even if the limited
docuemtnation that exists is enough, you would have to write 2 programs
(or add all sorts of conditionals to one program) -- one to handle RX01s
and PERQ double-density disks saves in Teledisk format, one to handle
RX02s archived in some other format.

No thanks. The whole point of a universal archive format is to only have
to do this sort of thing once.

> >So what happens in 20 years time when you want to recover the data from
> >one of these Teledisk archive files, and when there are no suitable PCs
> >left running
> I have a 20+ plus year old PC that runs just fine. I've passed up

So do I, I am using it right now. However in my experience that more
complex the chip, the more likely it is to fail (this doesn't mean that
the overall system reliabity goes down -- it doesn't because there are
fewer components to fail in a more modern machine with high levels of
integration). But I would suspect the 74xx chips on my PC's motherboard
will last longer than the gate arrays in a modern machine.

> HUNDREDs more that also worked fine. In fact, I've seen almost none that
> didn't work.

You nave been _very lucky_

> (PC hardware is notoriosuly badly docuemnted and therefore
> >difficult to maintain
> Bah! The PC is probably THE best documented computer out there. Name
> another company that produced as thorough a Tech Ref as IBM. Name another

OK, trivial...

If you want both hardware and software documentation (i.e. schematics
_and ROM source code_) then the obvious ones are :

Apple : Apple ][ and Apple //e (and maybe other machines in that family).
I am told the IBM PC Techref was modelled on the Apple manuals.

HP : HP71B. I am sure you've read the complete IDS (I certainly have,
it's excellent). Incidentally, do you have the HP71 Forth IMS (Internal
Maintenance Specification)? It's very interesting.

Research Machines : RML380Z. It's a separate hardware 'Information File'
giving schematics, etc and a COS (Cassette Operating System) Source
listing. Admittedly I've never seen the latter (it was an optional
purchase), but it existed.

DEC : PDP11. The PDP11/45 CPU technical manual and printset (schematics)
are much better than the IBM Techref. You could get a source license for
the OS if you could afford it, too.

DEC : PDP8. I am not sure about the OS sources, but the hardware manuals,
at least for the 8/e are again excellent.

HEath/Zentih : H89/Z90. The manuals I got with my Z90 had schematics and
ROM source listings in them.

If you just want hardware docs (i.e. full schematics) and the programming
interface (i.e. not full ROM sources) then also :

HP : Portable+ , HP150, etc (and I assume the HP110). There are excellent
technical manuals for these machines

Acorn : BBC Micro, Archimedes, etc. Again, schematics are easy to obtain
either in 'Advanced User Guides' or service manuals

Raido Shack : All TRS-80s had service manuals available to anyone who
wanted to order them. And many had technical reference manuals available
as well.

Need I go on?

> comany that has produced and PUBLICLY sold as many service and parts

IBM service manuals (at least the Hardware Maintenance and Service manual
i have for my PC/AT) are boardswapper guides. They're pretty useless now,
they are totally useless when there are no more boards to swap.

> Name another computer with as many choices of OS and as many versions,

This has nothing to do with being able to maintain the machine.

> including third party and public domain OSs. Name another computer that has
> had THOUSANDS of third party books written about it. Name another computer

And most of those books are not worth the paper they're printed on!

> that has as much third party hardware and software support. List another
> computer that you can go to ANY school in the modern world and take classes

And do any of said classes actually cover the hardware at the chip level?

> on. Name another computer that was build with all standard TTL parts like
> the original PC was. Finally name another computer that was sold in the

Firstly the PC wasn't built entirely from TTL. There are some standard
Intel chips (CPU, DMA controller, interrupt controller, etc). And of
course programemd ROMs.

If we allow those, then a partial list would be Apple ][, TRS80 Model 1,
Model 3, Model 4, Sinclair ZX80, Nascom, HH Tiger, TRS-80 CoCo 1, Dragon
32, Sage 2, Apricot, Sirius, etc, etc, etc

If you allow PALs, then you could even add the early Apple Macs to that
list, along with the PERQ 3a, Torch XXX, etc

If you want to move the goalposts and not allow LSI processors, then how
about PDP8/e, PDP11/10, PDP11/34, PDP11/45, (etc), Philps P850, Philips
P854. PERQs (all except for the 3a, although admittedly there are LSI
chips -- but not custom ones -- things like a Z80, NEC 765, etc -- on the
I/O card), HP9810/20/30, and many, many, more.

> huge quantities and that you can find on EVERY continent like you can a PC.
> You just don't like PCs and MS-DOS! You've told me that face to face


> personally. Sure there are other computers that are smaller, bigger,
> faster, that do X better, etc, etc, etc but in terms of just plain
> availability and usefullness I'd pick a PC everytime.

Fine... You use a PC, that's yuor choice. However, I am not so arrogant
as to want to impose my choices on the rest of the world. In other words,
the whole point of a universal archive format (let's not forget what this
thread is really about) is that you don't have to use a particular
machine. If you want to use a PC, fine, use a PC. You can write software
to create and process that archive format. If I want to use a PERQ, well,
I expect to have to write the software myself, but I can still do it. If
Sellam wants to use his Apple ][ (and can therefore read disks that
neither the PC or the PERQ can read) then he can do that.

> >Sorry, but if I want to achive data, I want the
> >format of the archive to be fully documented so that I can recover it on
> >whatever machine I have access to.
> Yeah, like you're really going to get full documentation for that
> CatWeasel card!

Actually, I was under the impression that the programming interface was
docuemtned!. I may not care to use the card due to lack of hardware info,
but that is not the same as saying that the card shouldn't be used, and
that any archiver format I come up with will delibrately exclude the

Received on Tue Aug 10 2004 - 16:41:57 BST

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