Solution - Hardwired Control

What is Hardwired Control?
Hardwired control is a method of control unit design where control signals are generated by fixed combinational logic circuits (gates, encoders, decoders, flip-flops) as Boolean functions of input variables: Opcode ($I$), Timing signals ($T$), Flags ($F$), and Clock. No control memory is used. Each control signal $C_k = f(I,T,F)$. It is fast but inflexible - any change requires physical rewiring.

Various Methods to Design Hardwired Control Unit:

1. State Table / Classical Method (Sequence Table + Logic)
2. Delay Element (Switching Delay) Method
3. Sequence Counter (Counter) Method - Most Popular
4. PLA (Programmable Logic Array) Method

Describe in Detail: Sequence Counter Method (with example):

This is the most widely used textbook method.

Hardware:

• A $k$-bit Sequence Counter (SC) / ring counter that counts clock pulses and generates timing states $T_0,T_1,T_2...T_{n-1}$ cyclically. Usually decoded to one-hot.
• An Instruction Decoder that decodes IR-opcode into $D_0...D_{m-1}$ lines.
• Combinational Encoder (AND-OR network / PLA) to generate control signals.
• Control logic also uses Flags and mode bits.

Working - Example: 3-instruction CPU (LDA, ADD, BUN):
Suppose SC counts $T_0$ to $T_3$. Control signals are:

• Fetch (common to all):
  $T_0: PC_{out} \to AR$
  $T_1: M[AR] \to DR, PC+1 \to PC$
  $T_2: DR \to IR, Decode$
• Execute depends on $D_i$:
  $D_0 T_3: M[IR_{addr}] \to DR$ (LDA)
  $D_0 T_4: DR \to AC$
  $D_1 T_3: M[IR_{addr}] \to DR$ (ADD)
  $D_1 T_4: AC+DR \to AC$
  $D_2 T_3: IR_{addr} \to PC$ (BUN)

Boolean expression: $AC_{load} = D_0 T_4 + D_1 T_4$

Sequence:

1. Initially $SC=0$. At $T_0$ fetch step.
2. After each clock, SC increments; decoder selects instruction-specific path.
3. At $T_{last}$ SC is cleared to 0 for next fetch, END signal generated.

Advantages of Counter Method: Systematic, easy to understand timing.
Disadvantage: Many gates, counter delay.

Other methods briefly:

• Delay Element: Chain of delay elements propagates control pulse sequentially to generate $T_i$ without counter - simple but delay not precise.
• PLA Method: All combinational logic fused into one PLA - compact, used in modern RISC hardwired CU for high speed.