Q2Computer Architecture

The Control Unit is the component of the CPU responsible for directing and coordinating the operation of all other processor components — the ALU, registers, memory interface, and I/O interface — by generating the precisely sequenced and timed control signals needed to correctly execute each stage of the instruction cycle: fetching the next instruction from memory (using the Program Counter), decoding the fetched instruction to determine what operation it specifies, and generating the specific control signals needed to actually execute that operation (such as enabling the correct ALU operation, or triggering a memory read/write at the correct time).

Significance of the Control Unit: without a properly functioning control unit, the various other functional components of the CPU (ALU, registers, buses) would have no coordinated mechanism directing when and how they should interact to correctly execute a program's instructions in the proper sequence — the control unit is what transforms a collection of individually capable but otherwise passive functional units into a coherently operating processor capable of correctly executing arbitrary programs. The control unit is also responsible for managing interrupt handling (recognizing interrupt requests and directing the processor to the appropriate interrupt service routine), and for managing the overall instruction pipeline in modern pipelined processors (ensuring instructions correctly progress through fetch, decode, execute, memory-access and write-back stages without incorrect interference between overlapping instructions). The control unit can be implemented using either a hardwired design (fixed logic circuits, generally faster but less flexible/harder to modify) or a microprogrammed design (a stored control-store 'microprogram' interpreting each machine instruction as a sequence of simpler micro-operations, more flexible and easier to modify/debug, generally at some performance cost compared to hardwired control), as discussed further in the following question.