1. The microprogram counter (MPC) register, used for repeat (reuse same address)
and continue (increment address by 1) instructions.
2. The stack, which supports microprogram subroutine calls as well as iterative
loops and returns from interrupts.
3. The DRA and DRB ports, which provide two additional paths from external
hardware by which microprogram addresses can be generated. These two
ports are connected to the most significant and least significant 16 bits of the
DA bus, respectively. This allows the microsequencer to obtain the next instruction
address from the WCS data field of the current microinstruction or
from a result calculated by the ALU.
4. Register counters RCA and RCB, which can be used for additional address
storage.
5. An external input onto the bidirectional Y port to support external interrupts.