Lecture Notes: Computer Architecture and Instruction Execution

Introduction

• Lecture focuses on computer architecture, specifically on the von Neumann model.
• Discussion on computing models, instruction set architectures (ISA), and logic levels.
• Introduction to digital design and high-level abstraction in computing.
• The von Neumann architecture is a key focus, leading to discussions on different components of a computer system.

Key Concepts in Computer Architecture

Von Neumann Model

• Components:
  • Memory: Stores both program and data.
  • Processing Unit: Executes operations.
  • Input and Output: Interfaces for external communication.
  • Control Unit: Manages the execution of instructions.
• Characteristics:
  • Stored Program Concept: Both program and data stored in the same memory.
  • Sequential Instruction Processing: Instructions are executed sequentially.

Memory

• Memory stores instructions and data in a linear manner.
• Address Space & Addressability:
  • Address Space: Total number of unique addresses available.
  • Addressability: Defines the size of data that can be accessed with a single address.

Processing Unit

• ALU (Arithmetic Logic Unit): Executes arithmetic and logic operations.
• Registers: Fast temporary storage close to the ALU, used for quick data access and storage.

Control Unit

• Orchestrates the step-by-step execution of instructions.
• Program Counter (PC): Keeps track of the address of the current or next instruction.
• Instruction Register (IR): Holds the bits of the current instruction being executed.

Instruction Execution

Instruction Cycle

• Phases of Instruction Execution:
  • Fetch: Retrieve the instruction from memory.
  • Decode: Determine what the instruction means and prepare for execution.
  • Evaluate Address: Calculate the memory address if needed.
  • Fetch Operand: Retrieve any necessary operands for the instruction.
  • Execute: Perform the operation specified by the instruction.
  • Store Result: Write the result of the operation back to the destination.

Types of Instructions

• Operate Instructions: Perform operations in the ALU.
• Data Movement Instructions: Load data from memory or store data to memory.
• Control Flow Instructions: Change the sequence of execution (e.g., branches/jumps).

Instruction Formats

• LC3 ISA: 16-bit instructions with different fields for opcodes and operands.
• MIPS ISA: Different instruction formats (R-type, I-type, J-type) with specific fields for operation and operands.

Examples and Encoding

• Instruction Encoding:
  • Specific bit patterns define different operations and operand specifications.
  • Example discussed: ADD and LOAD instructions in LC3 and MIPS.

Addressing Modes

• Base + Offset: Calculate address using a base register and an offset (immediate value).
• Immediate: Operands are specified directly within the instruction.

Practical Aspects

• Registers and Memory Usage: How these elements are used in executing instructions efficiently.
• Control Logic and State Machines: Finite state machines orchestrate the execution cycle by generating control signals.
• Hardware Implementation and Instruction Cycle: Example microarchitecture (LC3B) showing practical implementation.

Conclusion

• Understanding of the von Neumann architecture and instruction execution is foundational.
• Application in designing computer systems, writing efficient programs, and understanding ISA specifications.

Additional Notes

• Different ISAs (e.g., LC3, MIPS) provide varying capabilities and design choices.
• Importance of sequential logic, multiplexers, and decoders in instruction processing.
• Exploration of operate instructions, data movement, and control flow in future lectures.