Hardwired Control Unit [CSO EC 504] Prof. Shruti Dixit

To execute an instruction, the control unit of the CPU must generate the required control signal in the proper sequence. There are two approaches used for generating the control signals in proper sequence as Hardwired Control unit and Micro-programmed control unit.
Hardwired Control Unit –
The control hardware can be viewed as a state machine that changes from one state to another in every clock cycle, depending on the contents of the instruction register, the condition codes and the external inputs. The outputs of the state machine are the control signals. The sequence of the operation carried out by this machine is determined by the wiring of the logic elements and hence named as “hardwired”.
• Fixed logic circuits that correspond directly to the Boolean expressions are used to generate the control signals.
• Hardwired control is faster than micro-programmed control.
• A controller that uses this approach can operate at high speed.
• RISC architecture is based on hardwired control unit

o A Hard-wired Control consists of two decoders, a sequence counter, and a number of logic gates.
o An instruction fetched from the memory unit is placed in the instruction register (IR).
o The component of an instruction register includes; I bit, the operation code, and bits 0 through 11.
o The operation code in bits 12 through 14 are coded with a 3 x 8 decoder.
o The outputs of the decoder are designated by the symbols D0 through D7.
o The operation code at bit 15 is transferred to a flip-flop designated by the symbol I.
o The operation codes from Bits 0 through 11 are applied to the control logic gates.
o The Sequence counter (SC) can count in binary from 0 through 15.