Magnetic tapes as a random access medium?

Question

A two-part question:

1. How widespread in legacy systems was the practice of using magnetic tapes as a genuinely random access medium at the OS level by pre-formatting them in a way before the first use, then an application referring to data blocks on tape by number, and the OS automatically issuing the appropriate number of Block Skip Forward or Block Skip Back commands before performing the actual block read or write operation?

   Update: That's DECtape, for example.

2. How widespread, if at all, was the practice of emulating such random access tapes with IBM 9 track tapes, when the original equipment went out of commission but the need for tapes with random access remained?

Answer 1

DECtape was used that way; it was wide (0.750 inch?) magnetic tape, on very small reels, so it didn't take forever to find a sector. PDP-8 and PDP-11 systems from Digital Equipment were the likely systems sporting DECtape drives. The tape was fully redundant, took a LOT of wear without losing data, and that made it suitable for such system-program use.

That would be about the late 1970s. As I recall, 1/2" tapes of that era were not used for any such OS functions, because hard disk drives weren't more expensive than a good full-reel tape transport. Nine-track and ANSI labeled standard tapes weren't as suitable for such service.

Answer 2

There's another use of tape drives that is "sort-of" random access and floppy-like, besides DECtapes: The Exatron Stringy Floppy and the ZX Microdrive used tapes that consisted of a closed loop in a cartridge, and was used for home computers (TRS-80 resp. Sinclair ZX Spectrum) as a cheaper alternative to floppy disks. The loop was transported much faster then the audio cassettes that were also used for storage, and used higher quality type, so access times and capacity were comparable to a floppy disk.

However, the tape was not pre-formatted, organziation was not by blocks, but by files, and there were no skip commands (the loop was read/written continously). So if you don't restrict your question to that particular way of organzing it, then yes, there were other kinds random access tapes as well. (I still wrote this up as an answer instead of a comment, because it's much too long for a comment).

And of course nobody thought about replacing them with IBM tape drives. :-)

Answer 3

The Atlas (Manchester University/Ferranti, started ~1958, in service 1962) used pre-formatted addressable tapes.

The tape mechanism used on Atlas is the Ampex TM2 (improved FR 300) using one inch wide magnetic tape. There are sixteen tracks across the tape - twelve information tracks, two clock tracks, and two tracks used for reference purposes. The tapes are used in a fixed-block, pre-addressed mode. Information is stored on tape in blocks of 512 forty-eight bit words, together with a twenty-four bit checksum with end around carry. Each block is preceded by a block address and block marker and terminated by a block marker; the leading block address is sequential along the tape, and what is effectively the trailing block address is always zero. Tapes are tested and pre-addressed by special routines before being put into use, and the fixed position of the addresses permits selective overwriting and simple omission of faulty patches on the tape.

It appears from the description that the supervisor got an interrupt as each block was read, i.e., "seek" was mediated by software.

Tapes were used for input and output spooling, and for the 'job well' (queue).

Link - see section 4

I wonder whether this is the first use of addressable magtape.

Answer 4

It is very unlikely this was ever done on legacy reel-to-reel tape machines, since the time to wind through a 2,400 foot tape to find the next block of data would have been ridiculously slow.

In commercial computing applications, the standard way to process a large dataset in "random order" was to start by sorting it into the required order. This could be done using sequential reading and writing with several tape drives. For example with three tape drives, you could first copy (unsorted) sections of the data onto several tapes, and then read a block of data from two tape drives, sort it in memory, and write it in sorted order onto a third drive. Repeating that "merge sorting" procedure enough times would sort an arbitrarily long data set.

In principle, the IBM S/360 tape drives could have done this, because they were capable of backspacing over a data block, and then reading or writing with the tape moving forwards. But writing new data in this way could cause problems, since the data blocks on tape could be of variable length, and obviously you could not replace a block by a longer one.

This "backwards reading" was used in some specialized applications - for example in solving systems of equations which were much larger than the available main memory, it was possible to write intermediate results onto a tape, and then read them backwards, while doing more calculations, and writing the final results to a second tape. I used software that worked like that in the 1970s.

There were various special-purpose storage devices that used many short loops of tape to achieve a reasonable random access speed, with much more storage capacity than the available magnetic disks (which often had a capacity of only a few Mb, not a few Tb as now) Once such device was the https://en.wikipedia.org/wiki/IBM_2321_Data_Cell.

Hardware failures on the 2321 and similar mechanical devices could be "entertaining" - the data could literally end up being shredded and scattered around the computer room floor!

Later, the https://en.wikipedia.org/wiki/IBM_3850 was a much larger scale version of the 2321, using tape cartridges storing about 50Mb with a robot device to mount the tapes onto drives and transfer the data to disk for access by the computer. By 1980, the largest model could store about 500Gb - but in a 20-foot-long cabinet, not a 2 inch diameter PC disk drive!

Answer 5

The Coleco Adam was an early home computer that used a pair of block-addressable cassette tapes as its secondary storage. I helped a friend develop a Forth interpreter for it, but the system was quickly outdated and it went nowhere.

Even at the time, everyone understood that this was a sub-optimal solution to the problem of block-addressability. There was never a "need" for these systems to be emulated; everyone migrated to disks (floppy or hard) as soon as it became feasible.