2.1.1 CPU and Machine Instruction Cycle Notes

CPU Architecture

The central processing unit (CPU) is the brain of a computer, responsible for executing instructions and processing data. It consists of several key components, each with a specific role:

1. Arithmetic Logic Unit (ALU): Performs arithmetic and logical operations.
2. Control Unit (CU): Manages the execution of instructions by directing the flow of data within the CPU.
3. Registers: Small, fast storage locations within the CPU, including the Memory Address Register (MAR) and Memory Data Register (MDR).
4. Buses: CPU connections to primary memory and other components.

A block diagram of the CPU illustrates the relationships between its components:

The Arithmetic Logic Unit (ALU)

The Arithmetic Logic Unit (ALU) is responsible for performing all arithmetic and logical operations. It is the computational engine of the CPU, handling tasks such as:

1. Addition, subtraction, multiplication, and division
2. Logical operations like AND, OR, and NOT
3. Comparison operations such as greater than, less than, and equal to

The Control Unit (CU)

The Control Unit (CU) is the orchestrator of the CPU, responsible for:

1. Fetching instructions from primary memory
2. Decoding instructions to determine the required operations
3. Executing instructions by coordinating the ALU and other components

Registers: MAR and MDR

They play a crucial role in the execution of instructions. Two important registers are:

1. Memory Address Register (MAR):
   1. Holds the memory address of the data to be accessed.
   2. Communicates with primary memory via the Memory Address Bus.
2. Memory Data Register (MDR):
   1. Holds the data to be transferred to or from memory.
   2. Communicates with primary memory via the Data Bus.

Buses

There are three main types of buses relevant to the CPU:

1. Memory Address Bus: Carries memory addresses from the MAR to primary memory.
2. Data Bus: Transfers data between the MDR and primary memory.
3. Control Bus: Transmits commands from the CPU and returns status signals from other components, ensuring proper coordination and operation.

The Machine Instruction Cycle

Why do we need it?

1. Efficiency: The cycle enables the CPU to execute instructions quickly and efficiently.
2. Scalability: The cycle is the foundation for more complex operations and multitasking.
3. Universality: All modern CPUs follow this basic cycle, making it a fundamental concept in computer science.

Let's see how it works step-by-step:

1. Fetch Instruction from Primary Memory to Control Unit

1. Identify the Next Instruction: The control unit (CU) determines which instruction to fetch next.
2. Access the Instruction: The memory address register (MAR) holds the address of the instruction in primary memory (RAM).
3. Transfer the Instruction: The instruction is sent from memory to the memory data register (MDR) via the data bus.
4. Load into the Control Unit: The instruction is transferred from the MDR to the CU for processing.

2. Decode Instruction in Control Unit

1. Interpret the Instruction: The CU decodes the instruction to understand what action is required.
2. Identify Required Data: The CU determines if additional data is needed from memory.
3. Fetch Additional Data: If necessary, the CU uses the address bus to retrieve data, which is then loaded into the MDR via the data bus.

3. Execute Instruction

1. Perform the Operation: The arithmetic logic unit (ALU) executes the instruction using the data provided.
2. Handle Additional Data: If more data is needed, the CU fetches it from memory.
3. Compute the Result: The ALU processes the data and produces a result.

4. Store Result of Execution and Check for Next Instruction

1. Store the Result: The result is placed in the MDR and sent to the specified memory address via the data bus.
2. Update the Program Counter: The program counter (PC) is updated to point to the next instruction.
3. Repeat the Cycle: The CPU checks for the next instruction and repeats the process.