PDP11 PeripheralsHbk 1972 - Trailing-Edge

Figure 3"2 - DECtape Block Arrangement
The blocks permit digital data to be partitioned into groups of words which are in"
terrelated while at the same time reducing the amount of storage area that would
be needed for addressing individual words. A simple example of such a group of
words is a program. A program can be stored and retrieved from magnetic tape in
a single block format because it is not necessary to be able to retrieve only a
single word from the program. It is necessary, however, to be able to retrieve dif·
ferent programs which may not be related in any way. Thus, each program can be
stored in a different block on the tape.
Since DECtape is a fixed address system, the programmer need not know accurately
where the tape has stopped. To locate a specific point on tape he must only
start the tape motion in the search mode. The address of the block currently pas·
sing over the head is read into the DECtape control and loaded into an interface
register. Simultaneously, a flag is set and a program interrupt can occur. The program
can then compare the block number found with the desired block address
and tape motion continued or reversed accordingly.
TU56 DECtape Unit
35