Organization of control memory

The control memory is a ROM (read-only memory) inside the control unit in which all the control information (the microprograms) is stored permanently. Each word is a microinstruction.

• Size: 2ⁿ words for an n-bit address; Mano's example has 128 words of 20 bits, with a 7-bit address.
• Organization of routines: it is divided into areas. In Mano's example, the fetch routine is at 64–66, the indirect routine at 67–68, and the execute routine of each instruction at the address given by 0 xxxx 00 (ADD = 0, BRANCH = 4, STORE = 8, EXCHANGE = 12), so each routine can have up to 4 microinstructions.
• Addressing: the control address register (CAR) gives the address of the microinstruction; the sequencer supplies the next address.
• Output: the microinstruction is placed in the control data (pipeline) register, whose fields give the control signals.
• Access: it is a very fast memory, so that it does not slow the control unit. A writable control memory (writable control store, WCS) can be used for flexible designs.

 CAR --> [ Address decoder ]--> [ ROM array: micro-routines ]
                                      |
                              Control data register
                                      |
                               control signals