Unit 1- Computer Science

• Purpose of the CPU:

  • The CPU processes instructions
  • When you run a program, it is the CPU which runs the instructions
  • It is often thought of as being the ‘brains’ of the computer
  • A CPU simply runs one simple instruction at a time
  • It carries out billions of instructions per second
• Stored program concept:

  • Before 1943, early computers stored data in memory but not the program
  • In 1943-44, the stored-program computer concept was introduced by von Neumann and colleagues
• Von Neumann architecture:

  • Program instructions and data are stored in the same memory
  • The CPU accesses both instructions and data from the same RAM
• Components of the CPU:

  • The CPU has two major components: Control Unit and Arithmetic-Logic Unit (ALU)
  • Registers are used to carry out operations
  • Registers are very fast memory locations in the CPU
  • Cache is located on the CPU, slower to access than registers but faster than RAM
• Registers in the CPU:

  • Program Counter (PC) holds the address of the next instruction to be executed
  • Memory Address Register (MAR) holds the memory address of the current instruction and data
  • Memory Data Register (MDR) holds the actual instruction and fetched data
  • Accumulator holds the result of an instruction before transferring to memory
• Control Unit:

  • Coordinates and controls all activities within the CPU
  • Decodes and executes instructions
  • Directs timing and control of other CPU parts
• Arithmetic Logic Unit:

  • Where arithmetic and logical operations are done
  • Performs operations like AND, OR, and NOT
• Processor architecture:

  • ACC (Accumulator) temporarily stores arithmetic results
  • ALU (Arithmetic Logic Unit) performs calculations and logic expressions
  • MDR (Memory Data Register) stores data to be sent/received from memory
  • PC (Program Counter) points to the next instruction
  • MAR (Memory Address Register) stores the address of the instruction to be executed
• FDE cycle

  • Fetch: next instruction and data fetched from main memory
  • Decode: instruction decoded
  • Execute: instruction is executed
  • Process is repeated