Control unit design refers to different methodologies for implementing the control unit in a CPU, which is responsible for coordinating the execution of instructions and controlling the flow of data within the CPU.
There are primarily two control unit design approaches:
Hardwired Control:
- In hardwired control, the control unit is implemented using combinational logic circuits (such as AND, OR, and NOT gates) and flip-flops.
- The control signals are directly generated by the hardware circuits based on the instruction opcode and the current state of the CPU.
- Each instruction opcode is decoded by dedicated logic circuits to determine which control signals to assert.
- Hardwired control is typically faster and more efficient in terms of execution speed because control signals are generated directly without the need for additional interpretation.
- However, designing and modifying hardwired control can be complex and time-consuming, especially for architectures with a large and diverse instruction set.
- The Intel 8086 microprocessor, released in 1978, is an example of a processor that employs hardwired control.
Microprogrammed Control:
- In microprogrammed control, the control unit is implemented using a microprogram stored in control memory (often a ROM or PLA).
- The microprogram is a sequence of microinstructions, each of which corresponds to a set of control signals for a specific operation or instruction.
- The instruction opcode is used to index into the control memory to fetch the corresponding microinstruction sequence.
- Each microinstruction specifies the control signals to be asserted during a particular clock cycle, including operations such as memory read/write, ALU operation, and register transfer.
- The IBM System/360 mainframe series, introduced in the 1960s, is an example of a processor family that utilizes microprogrammed control.
Comparison of hardwired control and microprogrammed control
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