18.2. Design of Hardwired Controlled Control Unit

Welcome to the 6th unit of the module. Here, we will be discussing mainly the design of hardwired control unit.

If you look at the last few units we are basically discussing that for a macro instruction, what are the micro-instructions; and for each micro-instruction what are the basic control signals required; and how they can be generated or for each given micro instruction what are the control signals to be generated; and what is the proper sequence for that.

In this introduction, we focus on the main objective of this unit: understanding the design of hardwired control units. A hardwired control unit generates specific control signals based on instruction codes called opcodes. In previous units, we learned about the relationship between macro instructions (high-level commands) and micro-instructions (low-level operations) required to execute them. Each micro-instruction has a set of control signals that the control unit needs to produce, and these signals determine the flow of operations in the computer's CPU.

Consider a factory assembly line where specific machines operate based on instructions from a central control system. Just like how a hardwired control unit sends specific commands to machines based on the type of product being made, the control unit in a CPU directs its components to perform tasks based on the instructions provided.

We have seen that for a given instruction; there is fetch, decode and execute there are sequence of micro-instructions corresponding to each phase and for each of the micro instruction there is a sequence of control signals to be generated like program counter out memory address register in set the read mode to the memory, then wait for the memory to give a signal that it is ready etcetera.

Control signals in hardwired units are generated in three main phases – fetching, decoding, and executing instructions. When an instruction is fetched, the program counter indicates where the instruction is located. The control unit must then decode that instruction and generate control signals that tell other components (like the memory address register) how to process the instruction. This involves setting the appropriate read/write modes and waiting for data as needed. Each micro-instruction corresponds to a specific sequence of control signals that ensures the CPU operates correctly.

Think of a waiter in a restaurant. When a customer places an order (fetch), the waiter understands it (decode) and then goes to the kitchen with the correct instructions (execute). In a similar fashion, the control unit processes instructions in defined steps, ensuring everything runs smoothly—just like a good waiter ensures the right food reaches the right table.

There are two types of manner in which we can generate the control signals: one is actually called the hardwired which we are going to do today and another is basically called micro program based.

There are two approaches to generate control signals in control units: hardwired and microprogrammed. Hardwired control units rely on fixed logic circuits to produce control signals, which results in a faster but less flexible system. In contrast, microprogrammed control units use software routines stored in memory to generate signals, offering more flexibility but generally slower performance. In this unit, our focus is solely on hardwired control units, which are dedicated systems using a finite state machine.

Imagine two types of traffic lights at a busy intersection. One type uses a fixed timer that changes the lights in a set sequence (like a hardwired unit), making it very efficient but inflexible to changing traffic conditions. The other type uses sensors to adjust the timings based on real-time traffic data (like a microprogrammed unit), offering more flexibility but requiring more processing time.

In hardwired, basically, what is going to happen; we will have a dedicated finite state machine which will move from one state to another. Each state will correspond to one time step of the micro instruction or one time step or one micro-instructions basically.

The heart of a hardwired control unit is its finite state machine (FSM). The FSM transitions between various states, with each state corresponding to a specific micro-instruction. As the CPU processes an instruction, the FSM moves from one state to another, triggering the appropriate control signals at each step. This structure ensures that the system systematically completes the operation by determining which signals are needed at each point of instruction execution.

Consider a robot performing a series of assembly tasks. Each task represents a state in the robot's finite state machine. As the robot completes a task, it moves to the next state, following a predefined sequence that ensures each part is assembled correctly. This is similar to how a finite state machine in a hardwired control unit processes instructions step by step.

Control step counter is nothing but if you have a finite state machine which goes from one step to another so, also you have to know that what are the values of the state variables at that state.

The control step counter is a crucial component within the hardwired design. It tracks which state the finite state machine is currently in, allowing the system to know which control signals to activate next. Each state may have specific input values that influence the generation of outputs, ensuring that the FSM produces the correct control signals at the appropriate times.

Imagine following a recipe in cooking: each step in the recipe is akin to a state in a finite state machine. As you follow the recipe, you keep track of which step you are on (that's your control step counter). If the recipe instructs you to check if the water is boiling (state), you do precisely that before moving on to the next step, just as a control step counter ensures that the CPU follows the correct sequence of operations.

The inputs are from the instruction register control flags and status registers and control flags some external signals and the control step counter; that is very important what is a control step counter it is nothing but if you have a finite state machine which goes from one step to another.

The hardwired control unit relies on various inputs like the instruction register, control flags, and external condition signals to function correctly. It interprets these inputs to control the output signals sent to different components of the CPU. Outputs may include signals to read or write operations in memory, manipulate data in the ALU, and more. Each output depends on the state of the control step counter and the signals from the inputs.

Think of a conductor in an orchestra. The conductor (the control unit) takes cues from the musicians (inputs) to ensure they play at the right time (generate outputs). In this role, the conductor must also keep track of the rhythm and cues (the control step counter) to maintain the harmony of the performance, just as the hardwired control unit orchestrates various signals to lead the CPU through processing tasks.

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