The CPU

• We have seen a basic CPU with wombat..
• The general components of a CPU are
  • Memory/Registers
    • These are built out of flipflops.
    • They are usually all the same size.
      • In most modern machines there are 64 bits.
      • But you may still have a 32 bit CPU.
    • There are two types of registers.
      • Special purpose registers, covered later.
      • General purpose registers.
    • General purpose registers hold temporary computation.
      Think of these as variables, or temporary memory
      • There are anywhere from 1 to 32 of these, but there could be more.
      • The programmer can save values in these registers.
      • Sometimes the collection of registers is called a register file (RF).
        • And are indexed from 0 to the size of the register file.
    • The program counter (PC) is a special purpose register.
      • This is sometimes called the instruction pointer.
      • This stores the address in memory of the next instruction to be executed.
      • This is normally incrementd each instruction to point to the next instruction.
      • But may be changed radically for a jump.
    • The instruction register (IR) is a special purpose register.
      • It holds the current instruction.
  • The Arithmetic Logic Unit (ALU) conducts all math.
    • This is where all computation takes place.
    • It can add, subtract, multiply, divide.
    • It can do comparison.
  • The Memory Management Unit (MMU) interacts with memory
    • It can read or write words to/from memory.
    • It is connected to an external bus.
  • The control unit (CU)
    • This looks at the instructions in the IR and decides what to do.
    • This controls all other parts of the CPU.
    • The clock is probably part of the CU.
  • The internal bus
    • Connects all of the components.
  • The fetch-decode-execute cycle
    1. fetch
       1. Fetch the instruction pointed at by the PC
       2. Store this instruction in the IR
       3. Increment the PC for the next instruction.
    2. Decode
       1. The CU looks at the instruction in the IR and "decides" what to do.
    3. Execute
       1. The CU provides signals to the individual components to carry out the instruction.
       2. This could include
          • Fetching data from memory, RF, or even IR
          • Performing a Mathematical operation
          • Storing data in memory, RF, or even PC