18.2.4. Process of Signal Generation in Finite State Machine

If we look at the unit summary; we know that basically there is a set of micro-instructions for a given set of macro instructions.

This section discusses that for every macro instruction in a controller, there exists a corresponding series of micro-instructions that need to be executed. Each micro-instruction is linked to specific control signals which dictate the operations that need to be performed by the CPU, such as fetching data from memory or executing arithmetic operations. Essentially, the design of signal generation revolves around understanding how these micro-instructions translate into control signals that are necessary for the completion of complex tasks by the CPU.

Imagine a chef in a kitchen (the CPU) following a recipe (the macro instruction). Each step in the recipe requires different actions like chopping, boiling, or mixing (micro-instructions), and each action needs specific tools and ingredients (control signals). Just like the chef references the recipe to know what to do next, the CPU refers to micro-instructions to generate the right control signals.

There can be two approaches: one is called hardwired and another one is called the micro programmed.

There are two main approaches for generating control signals in a CPU: hardwired and microprogrammed. In hardwired systems, a fixed set of circuits and logic gates is used to produce control signals directly from the inputs, meaning the control signals are generated quickly but with little flexibility. In contrast, microprogrammed control units use a more flexible software approach, where control signals are generated based on a stored set of microinstructions. This allows for easier updates and more complex operations, although it often operates at a slower pace compared to hardwired systems.

Think of hardwired control as a car with a specific set of gears (like those fixed circuits): it’s quick but can only drive in certain conditions. Meanwhile, a microprogrammed control car has an automatic transmission, allowing it to adapt to different driving conditions and provide a smoother experience, albeit at a potentially slower response 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.

In a hardwired control unit, a finite state machine (FSM) is established. This FSM is designed to progress through various states, where each state represents a specific time step of a micro-instruction. As the machine transitions from one state to another, it generates the needed control signals associated with each micro-instruction step. This approach requires a thorough understanding of the rules governing state transitions, as these determine how the system responds to different inputs.

Consider a simple traffic light system as a finite state machine. The traffic light has distinct states (green, yellow, red) and follows a set sequence to switch between them. The signals (like when to stop or go) generated by each light state dictate how vehicles should react, just like how control signals dictate CPU operations.

The inputs are from the instruction register, control flags, and status registers.

The finite state machine relies on multiple inputs including data from the instruction register, which contains the current operation to be performed, and control flags that signal certain conditions (like zero or carry). These inputs dictate the FSM's current state, and the corresponding outputs are generated control signals that dictate actions taken by the computer's ALU and memory units.

Think of the FSM as a teacher (the CPU) who bases lessons (outputs) on students’ requests (inputs). The students (inputs) indicate what they need help with (e.g., math or science). Depending on their requests, the teacher prepares different lesson plans (control signals).

Whenever you are going to take some concrete examples which will make this step absolutely clear.

The workflow describes how the finite state machine processes inputs step by step. When a particular instruction is received, the FSM enters a corresponding initial state, and based on the control step counter, it sequences through the defined states, generating the necessary control signals for each step of the micro-instruction. This process is repeated for each instruction, minimizing errors and ensuring the correct sequence of operations.

Imagine a factory assembly line. Each station on the line (state) has a specific role in assembling a product based on inputs (raw materials). The control step counter acts as a supervisor directing the workers (FSM) through each station (state), ensuring that each part is added in the proper order to produce a finished product.

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