Call for information on virtual tape formats 353

One matter-of-interpretation is that some DEC OS's (for example RT-11 prior to V5.5 or so) write the tape labels with 512-byte blocks instead of the usual 80-byte blocks when doing ANSI labeled tapes. The way the standard was interpreted, was that the standard didn't actually say that the block has to be 80 bytes, only that it specified what was in the first 80 bytes.

Just to confuse things even more, some DEC OS's (like VMS) insist that ANSI labels be only 80 byte records, so moving a tape between DEC OS's sometimes required some work (depending on version numbers...)

There was also some liberal interpretation of the standard to allow boot blocks to go at the beginning of the tape while the tape still being ANSI-ish.

Most minicomputer OS's had limits about how long physical and logical records could be. And there are occasional tapes that use ridiculously long physical records (megabytes!). TPC, with a 2-byte record length, doesn't deal well with these super-long records.

Another gotcha, outside the realm of ANSI labeled tapes, is DOS-11 tapes. DOS-11 tape label records are just 14 bytes long, and some tape hardware does not do well with records so short. As long as you never go back to real physical tapes and always stick with tape images, I don't think there's a problem.

Tim.