Crash Course Computer Science: CPU Programming

Introduction

• Presenter: Carrie Anne
• Last episode, discussed how to build a CPU from components like ALU, control unit, memory, and clock.
• CPUs are powerful because they are programmable, meaning software can change the tasks they perform.

Basic CPU Instructions

• Instructions include an operation code (opcode) and address or registers.
• Example program:
  • LOAD_A 14: Load value from memory address 14 into Register A.
  • LOAD_B 15: Load value from memory address 15 into Register B.
  • ADD B A: Add values in Register B and A, store result in Register A.
  • STORE_A 13: Store value from Register A into memory address 13.
• Program adds two numbers together.

Advanced Instructions

• Subtract function introduced.
• JUMP instruction: Changes program's execution flow.
  • Example: JUMP 0 returns to the beginning of the program.
• JUMP_NEGATIVE: Jumps only if ALU's negative flag is set.
• HALT instruction: Stops CPU processing, important for distinguishing between instructions and data.

Program Execution Example

• Modified starting memory values to demonstrate program flow.
• Described step-by-step execution including loading, adding, storing, and jumping.
• Introduction to infinite loops and conditional jumps, e.g., JUMP_NEGATIVE.

Loop and Conditional Programming

• Demonstrated how loops and conditions work with another example.
• Showed using SUBTRACT and JUMP_NEGATIVE to create program logic.
• Discussed calculating remainders and potential for more complex programs with software.

CPU Limitations and Solutions

• Hypothetical CPU constraints:
  • Limited to 16 instructions and 16 memory locations due to use of 4-bit opcodes and addresses.
• Real CPUs use:
  • Larger instruction lengths (e.g., 32 or 64 bits).
  • Variable length instructions: Using immediate values stored behind instructions.

Real CPU Example

• Intel 4004 processor (1971): First single-chip CPU.
  • Supported 46 instructions, including JUMP, ADD, SUBTRACT.
  • Used 8-bit immediate values.
• Modern CPUs like Intel Core i7 have thousands of instructions, with instruction sizes from 1 to 15 bytes.

Conclusion

• Modern processors have evolved with many added functionalities, discussed in next episode.