8 inch disk with 27 sectors?

ACMS Collection
1

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?

Capture
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2

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

3

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.

https://www.dropbox.com/scl/fo/x6zf2k2p72nakcnthgrkr/h?rlkey=ohhe45f68j2d9mmcdwv286uv9&dl=0

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

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4

Thanks for popping that up there! Had a poke around Disk #44 (one of the mysterious 27 sector disks) and found this:
CALTECH-DECUS RT11 FORTH 18-OCT-78
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.

5

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

6

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?

7

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

8

Hmmm Interesting, Time to pull out simh!

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

9

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

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