Differentiate between hardwired and micro programmed control unit. Explain each component of hardwired control unit organization.

Hardwired versus microprogrammed control unit

Components of the hardwired control unit

• Instruction register (IR): it holds the instruction. The opcode part is sent to the decoder.
• Instruction decoder: a decoder, for example 3 to 8, converts the opcode to one of the lines $D_0$ to $D_7$. Each line stands for one instruction type.
• Sequence counter (SC) and timing decoder: SC counts the clock pulses, and a 4 to 16 decoder produces the timing signals $T_0$ to $T_{15}$. Each signal marks one step of the instruction. At the end, SC is cleared.
• Flip-flops and flags: the mode bit I, the interrupt flip-flop R, and the status flags like zero and carry also affect the signals.
• Control logic gates: a network of AND, OR and NOT gates combines the decoder outputs, the timing signals and the flags. For example, a load signal for AR is given by

$$AR\text{ load} = R'T_0 + R'T_2,D_7,I'$$

• Control outputs: these go to the registers, the ALU, the bus selectors and memory, and they decide what is done in each clock pulse.

 IR -> [Decoder] -> D0..D7 --+
 Clock -> [SC] -> [T0..T15] -+-> [Control logic] -> signals
 Flags ----------------------+

The unit works at gate speed, but a change in the instruction set means a new circuit.

Explanation of the difference. In the hardwired unit, the signals are made by gates, so the speed is high but the circuit is fixed. In the microprogrammed unit, the signals are read from memory as microinstructions, so it is slow but easy to change. The first suits RISC, and the second suits CISC. A hardwired design needs a full redesign when a new instruction is added. A microprogrammed design needs only a new microroutine.