Microprogrammed Control Unit for the Simple Computer

• In the case of a hard-wired control unit the control matrix is responsible for sending out the required sequence of signals. 
• Each bit of a microinstruction might correspond to one control signal. 
• If the bit is set it means that the control signal will be active; if cleared the signal will be inactive. 
• Sequences of microinstructions could be stored in an internal "control" memory. 
• Execution of a machine language instruction could then be caused by fetching the proper sequence of microinstructions from the control memory and sending them out to the data path section of the computer. 
• A sequence of microinstructions that implements an instruction on the external computer is known as a micro-routine. 
• The heart of the controller is the control 32 X 24 ROM memory in which upt to 32 24-bit long microinstructions can be stored. 
• Addresses provided to the control ROM come from a micro-counter register 
• The micro-counter, in turn, receives its input from a multiplexer which selects from : (1) the output of an address ROM, (2) a current-address incrementer, or (3) the address stored in the next-address field of the current microinstruction. 
• The controller's address ROM is fed by the outer computer's instruction register. The address ROM maps the op-code of the instruction currently contained in the op-code field of the instruction register to the starting address of the corresponding microroutine in the control ROM. 

• The 5-bit CRJA (Control ROM Jump Address) sub-field holds a microinstruction address. Thus, the address of the next microinstruction may be obtained from the current microinstruction.