The direction of the cell vector rotation induced
film signal on the sense/digit line will determine what
was the recorded state of the film. As the film is read,
the cell magnetic state vector is forced in the hard
direction. If the transverse field is removed, the film
would fall back to one of the two easy states, a one or a
zero. The actual state that a film would return to,
however, would be uncertain, and a small amount of
demagnetization of the film may occur. For this reason,
reading a film in this manner is considered destructive
readout. To ensure the film returns to its original state,
the computer has an automatic hardwired restore
operation, which is the same as writing into film
memory. This operation is an internal operation and is
not controlled by the software user.
WRITE (RESTORE) CYCLE.— Film memory is
like core memory; the data read from film must be
written back into the film for permanent storage. This
portion of the storage cycle is known as the write or
restore operation. Each storage cycle consists of a read
and a write or restore operation. In writing information
to a film spot, a longitudinal field is required in addition
to the transverse field to assure proper writing. Frame
A of figure 6-23 shows how this field is applied. This
longitudinal field is applied by passing a current
Figure 6-23.—Writing (restoring) a zero or a one: A. How the field is applied; B. Bit steering to store a zero; C. Bit steering to
store a one.
6-19
