How Do Special Registers Work in the CPU? MAR, MDR, ALU, CU

Summary

This video explains the role of special purpose registers in a More Newman CPU, focusing on key components such as the program counter, accumulator, arithmetic logic unit, and memory registers. It demonstrates how data flows through the CPU during a program cycle, with detailed examples of instructions being fetched, decoded, and executed. The video also touches on how different registers work together to manage memory access and perform arithmetic operations. Viewers are introduced to basic machine code operations and the crucial role of the program counter in looping instructions.

Takeaways

• 😀 The video explains the role of special purpose registers in a More Newman CPU, including key components like the program counter and the accumulator.
• 😀 The CPU’s clock controls the cycle, fetching instructions, decoding them, and executing them sequentially.
• 😀 Special purpose registers like the memory address register (MAR) and memory data register (MDR) are crucial for accessing and transferring data between the CPU and RAM.
• 😀 The control unit decodes instructions and allocates resources to ensure the correct execution flow of the CPU.
• 😀 The arithmetic logic unit (ALU) performs mathematical operations and logical comparisons, and is responsible for the actual computation in the CPU.
• 😀 The accumulator is part of the ALU and holds values for computation, receiving data from the MDR and being used in arithmetic or logic operations.
• 😀 The memory address register (MAR) helps the CPU find specific locations in memory by storing the current memory address.
• 😀 The memory data register (MDR) serves as an intermediary between the CPU and RAM, holding data being transferred either into or out of the CPU.
• 😀 A simulation of the cycle shows the step-by-step execution of fetching an instruction, decoding it, and executing it, along with memory address handling.
• 😀 The program counter is incremented after each instruction fetch, ensuring the CPU progresses to the next instruction, except in cases where a jump instruction is used to create loops in the program.

Q & A

What are the special purpose registers mentioned in the video?

The special purpose registers mentioned include the program counter, current instruction register, accumulator, memory address register, and memory data register.

How does the control unit contribute to the CPU's operation?

The control unit decodes instructions and allocates resources, controlling the flow of information in the CPU.

What role does the arithmetic logic unit (ALU) play in the CPU?

The ALU performs arithmetic operations and logical comparisons. It is responsible for the actual computation in the CPU.

What is the function of the memory address register (MAR)?

The MAR holds the memory address that the CPU needs to access, guiding the CPU to the correct memory location to fetch instructions or data.

What is the purpose of the memory data register (MDR)?

The MDR acts as a buffer between the CPU and RAM, transferring data between them. It ensures that data passes correctly during read and write operations.

Why must the program counter (PC) be incremented after each instruction?

The PC must be incremented to ensure that the CPU points to the address of the next instruction in the sequence for execution.

How does the CPU fetch instructions during the cycle?

The CPU fetches instructions by first loading the memory address into the MAR, then fetching the instruction from RAM into the MDR, and finally moving it to the current instruction register.

What is the purpose of the 'load' instruction in the example program?

The 'load' instruction is used to load the contents of a specific memory address (e.g., address 30) into the accumulator for further processing.

What happens during the 'add' instruction in the example program?

During the 'add' instruction, the CPU adds the value from a specified memory address (e.g., address 31) to the value already stored in the accumulator using the ALU.

Why is the 'store' instruction important in the example program?

The 'store' instruction is used to write the value stored in the accumulator back to a specific memory address (e.g., address 30), effectively saving the result of computations.

How does the program create a loop using a jump instruction?

The jump instruction changes the value of the program counter, allowing the CPU to loop back to a specific memory address (e.g., address 0), thereby repeating the sequence of instructions.