Multi-platform distribution format (Was: Backups [was

[Johnny Billquist]

Not that I care what Holm Tiffe smokes, but I can at least comment what 
you write, Tony. :-)

On 2015-09-21 16:02, tony duell wrote:
>
> [Russian PDP11-a-like]

All bets are off when we talk about clones, since they might be rather 
different in details...

>> I've tried to boot an RX01 Floppy in RX02 mode, that failed on all disks
>> I've tried. Is an original RX02 able to boot (RT11) from an RX01 disk?
>
> As far as I know, it can't. An RX02 can read/write an RX01 disk, but the software
> interface to the controller is so different (the RX)2 uses DMA, the RX01 doesn't for
> one thing) that the RX02 cannot boot an RX01 disk.

Of course an RX02 *drive* can read an RX01 *floppy*. And yes, a system 
with an RX211 and an RX02 *drive* can boot from a floppy that what 
written by an RX01 *drive*. But you need the boot code for an RX211 
controller, and that boot code also needs to handle RX01 format 
floppies. If the boot sector on the floppy is for an RX11, then no, an 
RX211 will not understand things. The programming model for the two 
controllers are different. You need different device drivers.

But this is really a question about the controller and the software 
running it, not the floppy or the drive.

>> I know that a different device driver has to be used  normally, but I had
>> no RX02 media and wasn't able to make one, since the machine has no other
>> drives connected and the card cage is mechanically not compatible (metric
>> dimensions including the fingers).
>> I know that in the RX02 Mode the Drive has an format track command that
>> doesn't exist in RX01 mode, that's documented. How about the RX02?
>
> The RX02 has a 'set density' command that can be used to reformat a
> formatted single density disk as double density.

Yes. And it's just flipping a bit in the sector header. And the drive 
changes between the densities for the data part of each sector. So you 
can actually have different densities on a per-sector basis with an RX02 
drive.

>> Is there somewhere a picture from the Conteroller PCB of the DEC RX02
>> (just to look at). The russian controller has a 2910 and four 2901 if
>> I remember correctly... (K1804VU1 and 4xK1804VS1)
>
> The RX02 drive contains a couple of 2901s and a few of the sequencer chips
> (I think 2911s, maybe 2909s). I've seen a 3rd party Qbus card that connected to
> normal Shugart drives and which could red/write RX02 disks, I think that had
> a couple of 2901s and a 2910 on it.

What people need to understand is that there are a bunch of controllers 
and combinations. If we just contain our self to Unibus controllers, we 
have two. The RX11 and RX211. The RX11 works with both the RX01 and 
RX02. However, an RX02 needs to be set in an RX01-compatible mode if 
used with an RX11 (a couple of dip-switches in the drive). And that also 
turns it into essentially an RX01 (can't deal with double density within 
the RX11 interface). The interface to an RX01 is different than the 
interface to an RX02, even though they use the same flat cable.

The RX211 can be used with both an RX01 and an RX02, as far as I can 
remember. But that implies that there would need to be some switch or 
jumper on the RX211 in order to interface it with an RX01 (or some 
automatic detection), and I can't remember seeing that. But it might 
just be my memory failing me.
But the RX211 works differently than the RX11, so from a program point 
of view, they are totally different.

The RX02 drive, then, in turn, can read and write both RX01 and RX02 
floppies. No special tricks are required. The drive will detect, for 
each sector, which format it is in, and will switch mode accordingly, 
and return the right about of data (RX02 sectors holds 256 bytes, while 
RX01 sectors holds 128 bytes). The software can thus also easily see 
what type of floppy it is reading.
RX211 device drivers obviously then can handle both RX01 and RX02 
floppies, when used with an RX02 drive. And from the software point of 
view, the sectors just contains different amount of data.

	Johnny