8 inch disk with 27 sectors?

Hi Gang,
Just started in flux imaging another box of the ACMS 8 inch Disk collection and I have come across a set of FORTH disks that have been formatted as 77 Tracks 27 sect/trk 128 bytes/sect. (Normal is 26 or less sectors)

The disks say SSSD on the front, but when I image them they are in fact formatted on both sides. The second side seems to have the sectors in reverse order. Its not a flippy though (Never heard of an 8 inch flippy), because the CRC are all ok.

Does anyone have a clue what system they may have belonged to?


Interesting for sure! Would love to dive into the data to see if there’s any clues.

Here ya go,

If ya wanna have a play and see what you can find out. Turns out 2 of them were normal 26 sector SSSD and 3 were 27 Sector DSSD or something weird.

I’ll leave the link active for a few weeks.

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Thanks for popping that up there! Had a poke around Disk #44 (one of the mysterious 27 sector disks) and found this:
There are a lot of references to RT11A

I don’t have much in the way of PDP11 skills or trusted emulators to try it on, but looks like a good place to start.

I should be a little clearer- the disk itself has a lot of mentions of RT11A…

RT-11 is an Operating system for DEC. One our our small PDPs has dual 8” floppy drives as is the RX-11?

@m_thompson - can you shed any light?

Yeah, one of the Disks is Labeled Forth System, So perhaps it is a Forth Implementation for RSX11 on a PDP.

Hmmm Interesting, Time to pull out simh!

Although some of those disks did have a lot of bad sectors.

There are many operating systems for the PDP-11, but only a few that can be built small enough to fit on a diskette. RT-11 even has several versions depending on how much memory is available. There is a detailed description on Wikipedia: RT-11 - Wikipedia

Comparing RT-11 to RSX-11 is a little difficult because many of the capabilities overlap. There is also a Wikipedia page for RSX-11 here: RSX-11 - Wikipedia