Copy protection in RX01 floppies

[x***@qmailcomq.com]

An RX01 floppy holds 256256 bytes (77 * 26 * 128). Its capacity in
512 byte blocks is 256000 which leaves two sectors inaccessible from an
application.
A boot loader, on the other hand, is able to read any sector it pleases
and can check the contents of these two hidden sectors or even have some
of its routines there. Booting from a copied floppy would then cause mysterious
crashes.

Was this scheme used in any of the PDP-11 software?

Thanks,
Leo

PS. Am I right that RX01 was the only device with media capacity that
is not a multiple of 512?

[Michael Moroney]

Post by x***@qmailcomq.com
PS. Am I right that RX01 was the only device with media capacity that
is not a multiple of 512?

If you are talking about DEC disk drives, the answer is no. Some DEC
systems (TOPS?) used 576 byte sectors. Certain DEC drives could be
formatted for either 512 byte sectors or 576 byte sectors.

I remember in college the CDC system used sectors of 640 six-bit
characters. CDROMs really use 2048 byte sectors but appear as 512 byte
sectors to VMS.

[x***@qmailcomq.com]

Post by Michael Moroney

Post by x***@qmailcomq.com
PS. Am I right that RX01 was the only device with media capacity that
is not a multiple of 512?

If you are talking about DEC disk drives, the answer is no. Some DEC
systems (TOPS?) used 576 byte sectors. Certain DEC drives could be
formatted for either 512 byte sectors or 576 byte sectors.

The question was about the media capacity, not block size. If media capacity
is a multiple of 512, all of it can be used as a (UNIX) file system
and accessed via the corresponding block device. (Of course, if the
physical block size is not a divisor of 512, writing is expensive.)
If media capacity is not a multiple of 512, even root is unable to make
an exact copy of the media.

Leo

PS. It was not 576 bytes, it was 128 words, I bet.

[Eric Smith]

Post by Michael Moroney
If you are talking about DEC disk drives, the answer is no. Some DEC
systems (TOPS?) used 576 byte sectors. Certain DEC drives could be
formatted for either 512 byte sectors or 576 byte sectors.
PS. It was not 576 bytes, it was 128 words, I bet.

He was referring to RA60, RA80, and RA81 drives being used on an HSC50
storage controller over a CI intercoonect from a DECsystem-10 or
DECSYSTEM-20. CI transfers were in units of 8-bit bytes. The drives/media
were available with either 512-byte or 576-byte sectors.

The DECsystem-10 or DECSYSTEM-20 used 576-byte sectors on these devices
to store 128 36-bit words per sector.

[glen herrmannsfeldt]

Post by x***@qmailcomq.com
PS. Am I right that RX01 was the only device with media capacity that
is not a multiple of 512?

IBM has a large selection of disk drives with media capacity
not a multiple of 512. You get to choose your own block size
on each track, but a full track is not a multiple of 512.

The 2314 has 200 cylinders of 20 tracks of 7294 bytes each.
The 3330 has 404 cylinders of 19 tracks (plus a servo track),
with 13030 bytes in a full track.

For slightly more modern drives, the 3380 has 47476 bytes/track
and the 3390 has 56664 byte/track. There are models of each
with different numbers of cylinders, from the 3380J with 885
cylinders, to the 3390-9 with 10017. There is also a 3390-27.

Note that many IBM file formats have 80 byte records, so
512 is not necessarily a good size.

-- glen