3 main parts of the CPU

• What the 3 main parts of the CPU are

  • Control Unit 
  • Arithmetic Logic Unit 
  • The Cache

  
• What the Control Unit does

  • The control unit is in overall control of the CPU 
  • Its main job is to manage the fetching , decoding and execution of program instructions by following the fetch-decode-execute cycle 
  • It controls the flow of data inside the CPU (to registers , ALU , cache) 
  • It also controls the flow of data outside the CPU (to main memory and input / output devices)

  
• What the Arithmetic Logic unit does

  • The ALU does all the calculations 
  • It completes simple addition and simple subtraction , compares the sizes of numbers and can do multiplications and divisions using repeated addition and subtraction 
  • It performs logic operations , e.g. AND , OR and NOT ,and binary shifts (Remember, computers process binary data) 
  • It contains the accumulator register

  
• What the Cache does

  • It is very fast memory in the CPU - it is slower than the registers , but faster than RAM 
  • It stores regularly used data so that the CPU can access it quickly the next time it’s needed 
  • 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

  
• The Cache compared to RAM and secondary storage

  • Caches have a very low capacity compared to RAM and secondary 
  • Caches are expensive compared to RAM and secondary storage

  
• Levels of cache memory

  • The different levels of cache memory are L1 , L2 and L3 
  • L1 is the quickest , but has the lowest capacity 
  • L2 is slower than L1 but can hold more , and L2 is faster than L3 but can hold less 
  • L3 is the slowest has the highest capacity

  
• Registers

  • The CPU contains various registers which temporarily hold tiny bits of data needed by the CPU 
  • They are quick to read / write to (Quicker than any other form of memory) 
  • The program counter , memory address register (MAR) , memory data register (MDR) , and the accumulator are all registers (You need to know about them)