The CPU

• CPU = Central Processing Unit
• The CPU is the brain of the computer system
• The CPU processes all of the data and instructions thtat make the system work
• The processing power of the CPU depends on different characteristics, like its clock speed, number of cores and cache size
• The CPU architecture describes the main components of the CPU, how they interact with each other, and with other parts of the computer system
• Von Neumann and Harvard are the 2 main types of architecture. However, you only need to know about Von Neumann
• CPUs contain 1000s of gold pins - some transmit data, others supply power to the CPU

  
• The CPU has 3 main parts
• CU = Control Unit
• CU - The contol unit is in overall control of the CPU. Its main job is to manage the fetching, decoding and execution of program instruction by following the fetch-execute cycle
• CU - It controls the flow of data inside the CPU (to registers, ALU, cache...) and outside the CPU (to main memory and input / output devices)
• ALU = Arithmetic Logic Unit
• ALU - Basically does all the calculation
• ALU - It completes simple addition and subtraction, compares the size of numbers and can do multiplications and divisions using repeated addition and subtraction
• ALU - It performs logic operations such as AND, OR and NOT and binary shifts - computers process binary data
• ALU - It contains the accumulator register
• Cache - The cache is very fast memory in the CPU. It's slower than the registers but faster than RAM
• Cache - It stores regularly used data so that the CPU can access it quickly the next time it's needed
• Cache - When the CPU requests data, it checks the cache first to see if the data is there. If not, it will fetch it from RAM
• Cache - Caches have a very low capacity and are expensive compared to RAM and secondary storage
• Cache - There are different levels of cache memory: L1, L2 and L3.
• Cache - L1 is quickest but has the lowest capacity
• Cache - L2 is slower than L1 but can hold more
• Cache - L3 is slower than L2 but can hold more
• The CPU contains various registers which temporarily hold small bits of data needed by the CPU
• Registers are really quick to read / write to, way quicker than any other form of memory
• Von Neumann came up with the modern design of a computer in 1945
• The Von Neumann architecture describes a system where the CPU runs programs stored in memory
• Programs consist of instructions and data which are stored in memory addresses
• The program counter (PC) holds the memory ADDRESS of the instruction for each cycle

  (check image)

  A) Program counter

  B) Accumulator

  
• The accumulator stores intermediate results of calculations in the ALU

  A) Program counter

  B) Accumulator

  
• One of the main features of the Von Neumann architecture is that it only uses one memory for both the data and the instructions
• (Arrows show flow of data)
• The Memory Address Register (MAR) holds any memory ADDRESS about to be used by the CPU. The address might point to a data or CPU instruction

  A) MAR

  B) MDR

  
• The Memory Data Register (MDR) holds the actual data or instruction. This may have been fetched from memory, or be waiting to be written to memory

  
• The memory holds the program instructions and the program data

  
• CPUs follow the Fetch-Execute cycle
• Essentially, all a CPU does is carry out instructions, one after another, billions of times a second
• The fetch-execute cycle describes how it does it
• FETCH - 1. Copy memory address from the program counter to the MAR