Microprogrammed Control Unit for the Simple Computer

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  • 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. 


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