View Full Version : 8" disk formats

May 6th, 2011, 05:00 AM
Hi all,

I have just managed to connect a couple of 8" Shugart drives via USB (http://forum.kryoflux.com/viewtopic.php?f=3&t=159) and read the data off them. The only disks I have on hand are labelled "DEC RT11 V5.4G BIN RX01 MAN" and reading them in, the data appears to be interleaved in chunks of 128 bytes.

I've had a look around but can't really find anything documenting these disk formats. Does anyone know the correct way of de-interleaving the data and/or what filesystem is in use on the disk, if any? A Linux app to extract all the files would be ideal.

The disk image can be downloaded at the above link (decrt11a.zip in the third post) if anyone is curious. I'm not sure whether these disks are double-sided (only one index hole) but the drive itself is definitely single-sided.

Any pointers would be much appreciated!

May 15th, 2011, 12:25 PM
RX01 is single sided, single density, no interleaving. Some time ago I made images of my MiniMINC disks to ImageDisk format (http://www.xs4all.nl/~fjkraan/comp/miniminc/floppyImages/). This image format stores interleaving per sector, so I know it is just 1. No idea how the file system is organized, is clearly isn't FAT-like :-) I looked at it with an binary file editor but I couldn't even find something that looked like a directory.

Lou - N2MIY
May 16th, 2011, 04:11 PM
RX01 and RX02 were not only used on pdp-11s. They were also used on pdp-8s. An OS/8 formatted RX01 has an interleave of 2. An OS/8 formatted RX02 has an interleave of 3.


May 16th, 2011, 04:24 PM
A real RX02 (MFM) floppy is also a bit different from the standard (IBM System/3) variety. It uses FM headers with MFM sector data (with some notable substitutions in the bit patterns to avoid false AM detection).

May 22nd, 2011, 04:28 AM
Yes, the Kryoflux guys have been scratching their heads over the RX02 disk I dumped for them, but they managed to read most of it and it turned out it had the game 'Dungeon' on it. There are a couple of issues with their decoder (which converts the flux transitions read by the drive head into bits and bytes.) They're not sure whether the disks are corrupted or if their decoder is buggy. Perhaps these 'notable substitutions' are what they're seeing? Are there any docs available on how that works?

May 22nd, 2011, 08:17 AM
Take a look at Tim Mann's code for his DMK utilities. He explains it quite well in the source.

...or hadn't it occurred to you that a lot of formats and code had already been developed years ago for the Catweasel?

June 4th, 2011, 05:52 PM
It had occurred to me, but unfortunately all this stuff is well before my time so I have no idea what any of it is called, let alone what to search for :-( It doesn't help either that these were the days where the more cryptic an acronym was the better...

At any rate I got hold of some more 8" disks to image and I was able to find a bootable RT-11 floppy, which once imaged came up nicely in the simh PDP-11 simulator. It looks like the interleaving is part of the filesystem as I can use this to read files off the other disk images, but I'll have a look for these DMK utilities and see what they can do too. Thanks!