An optional in-out control is available for PDP-3. This control, termed
the Sequence Break System, allows concurrent operation of several in-out
devices and the main sequence. The system has, nominally, 16 automatic
interrupt channels arranged in a priority chain.
A break to a particular sequence may be initiated by the completion of
an in-out device, the program, or an external signal. If this sequence
has priority, the C(AC), C(IO), C(PC), and C(IA) are stored in three
fixed memory locations unique to that sequence. Since the C(PC) and
C(IA) are eighteen bits each, these two registers are stored in one
memory location. The next instruction is taken from a fourth location.
This instruction is usually a jump to a suitable routine. The program is
now operating in the new sequence. This new sequence may be broken by a
higher priority sequence. A typical program loop for handling an in-out
sequence would contain 3 to 5 instructions, including the appropriate
iot. These are followed by load AD and load IO from the fixed locations
and a special indirect jump through the location of the previous C(PC).
This special jump also loads the IA. This last instruction terminates
the sequence.