Information online states that a 18/36-bit DECtape holds 576 blocks. People claim it may be possible to add a few more blocks, but no more. Where does this number come from? Is there a hard limit on the length of the tape?

  • 1
    There's certainly a limit on how much tape can fit on one spool... given that the spools have a standard size. I do not know if there was wiggle room to replace the tape with a slightly longer tape, and still have it fit on the spool.
    – dirkt
    Aug 28, 2020 at 9:49
  • Can formatting be done "one the machine", and is that a hardware function?
    – dave
    Aug 28, 2020 at 12:45
  • @another-dave not sure what you mean, but e.g. on the PDP-8, a special program was used to format the type, and the hardware interface is generic, it doesn't have an opaque "format" function. So in principle, you can change block count, block length, etc.
    – dirkt
    Aug 28, 2020 at 18:58
  • 1
    A PDP-10 could format a DECtape. There was a manual switch labelled WRTM that had to be set to on before formatting could be done. WRTM stood for something like Write Timing and Mark track. Normally off. Aug 28, 2020 at 20:56
  • Ah, I couldn't remember the details, and in particular I wasn't sure whether the timing marks could be written "on site". Apparently they can. PDP-8 info here
    – dave
    Aug 28, 2020 at 21:00

1 Answer 1


The standard tape reels had a capacity of 260 feet of tape. Standard tape thickness was 1.25 mil, so it would have theoretically been possible to put more tape on a reel if you made it thinner (which was hard to do because thinner tape was typically also stretchier). Making the reel larger wasn't a good option because the hubs on the drives were too close together to allow them to be much larger than they were already, and you would have needed two larger reels, one for the tape and a second one to use as the take-up reel.

The reason you could fit a few extra blocks on the tape was because both ends had a lot of leader tape so that you could securely wind it onto the take-up reel. You could therefore squeeze a few more blocks on the tape if you were willing to (a) reformat the tape, and (b) have a slightly increased risk of tape not winding properly because there wasn't enough on the take-up reel to hold securely.

I believe the 'hard' limit on the number of blocks would have been 4096 because block numbers were recorded as 12-bit values. This is assuming there's nothing in the electronics that would have limited it to something smaller, or which treated block numbers as signed values, either of which would have reduced the maximum number of blocks.

The operating system or software in use might also have imposed a limit, for example any system that treated the tape as a slow disk might assume the default number of blocks, and therefore be unable to use the extra blocks without modifications.

  • I'm not sure you could have made the tape (much) thinner. Unlike vacuum-column drives DECtape drives directly pulled the tape over the head (in both directions), tugging on the other side. Had to be taut to keep the tape firmly on the heads, and there was no slack in vacuum columns, so the "rear" hub couldn't spin free - it had to provide tension. If you didn't want the tape to stretch - or even break - it needed to be pretty tough. (Plus, those of us who hung DECtapes on drives weren't really delicate about it ...)
    – davidbak
    Aug 28, 2020 at 17:34
  • The two layers of Mylar which had the magnetic medium sandwiched between them also gave less opportunity to reduce the thickness. Did wonders for reliability however.
    – Brian
    Aug 28, 2020 at 18:29

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