Explain the design and working of a hardwired control unit. Discuss about its advantages and disadvantages.

Hardwired control unit

Design

 IR (opcode) -> [Instruction decoder] -> D0 ... D7 --+
                                                     v
 Clock -> [Sequence counter] -> [Timing decoder] --> [Control logic] -> control signals
              ^   T0 ... T15                          ^   (AND, OR gates)
              |                                       |
         clear (end of instruction)           Flags, mode bit I, interrupt R

The parts are:

• Instruction register and decoder. The opcode bits go to a decoder, for example 3 to 8. It makes one line $D_0$ to $D_7$ high for the current instruction.
• Sequence counter and timing decoder. The counter counts clock pulses. A 4 to 16 decoder makes the timing signals $T_0$ to $T_{15}$. Each signal gives one time slot for a micro operation.
• Control logic network. AND and OR gates combine the decoder lines, the timing signals and the flags. There is a Boolean equation for each control signal. For example, the signal to load AR is

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

• Flags and condition bits. The zero flag, the carry flag and the mode bit I affect some of the signals.

Working

At $T_0$, $T_1$ and $T_2$ the same fetch signals appear for every instruction. After decoding, the line $D_i$ of the instruction and the later timing signals $T_3$, $T_4$, ... create the signals for the execute part. At the end, a signal clears the sequence counter, and the next instruction is fetched.

Advantages

• Very fast, since the signals come straight from gates.
• Good for RISC and pipelined designs.
• No control memory is needed.

Disadvantages

• The design is complex for a large instruction set, as every signal needs its own equation.
• It is hard to change. A new instruction means a new circuit.
• Error fixing is difficult and costly.
• Less regular, and it takes a long design time.

Use. Hardwired control is chosen in RISC processors, where the instructions are few and simple, so the equations are small and the speed matters more than flexibility.