Reading gapless tape

Field-recorded data are often most conveniently recorded on magnetic tapes without interblock gaps. Gapless tapes cannot be read by certain types of computers. A simple modification to a KDF 9 computer enables it to transcribe from gapless to conventional format as a time-shared monitor operation. The counter which increases peripheral transfer addresses and detects the completion of a block is time-shared by all channels on the KDF 9. It was modified so that for any channel which, in requesting a core memory access, indicates that it is engaged in a gapless read" carries out of the 64's digit of the counter are suppressed, and any change of the 64's digit causes an interrupt to the permanently resident monitor program (Director). Thus, if an input transfer is set up on a channel with initial core memory address an exact multiple of 128, and the channel is made to signal that it is reading gapless tape, the reading will proceed indefinitely, repeatedly cycling over the same 128 word core area, and causing an entry to Director after every 64 words.