Sector Interleave

When I attempted to evaluate how well a backup to tape would work, I always included the verify portion to make sure that the tape had been written correctly in addition to making sure that the tape could also be read. For the PDP-11/83 under RT-11, there were essentially two choices: TK50 and TK70. Aside from the advantage that the TK70 had over the TK50 with respect to the capacity - (if I remember correctly) about 256 MB vs 32 MB, the other problem was that it was found to be impossible to steam the TK50 during a BUP verify operation. So in addition to the TK70 being much faster in the first place relative to the TK50, the verify operation was just not feasible. Somehow, the PDP-11/83 could read both the tape and the disk drive, then compare the two current buffers while the alternate buffers were being read, using, I presume, DMA requests to the two controllers to transfer the data to the next pair of buffers. Of course, RT-11 had special EMT requests to initiate the read requests and then go away and do something else. Obviously, I could have copied the BUP backup to a scratch disk and verified the scratch disk against the original disk and saved a great deal of time if the TK50 had been the only option. Jerome Fine