Structure and function of a processor

Structure and function of a processor

1. CPU

   • The CPU (Central Processing Unit) is the brain of the computer. It does all of the computations.
   • As the CPU performs many different functions you need to divide it up into the main parts to understand it.

2. Von Neumann Architecture

   • Arithmetic Logic Unit (ALU)
     • Carries out the calculations and logical decisions. The results of the calculations are stored in the accumulator.
       • It can perform arithmetic calculations on data.
       • It can perform logical operations on data.
       • It can hold the data that has already been worked on and is ready to be processed. All the data that goes in and out of the CPU goes via the ALU.
   • Control Unit (CU)
     • Sends out signals to coordinate how the processor works. It controls how data moves around and between the processor and the memory.
   • Registers
     • These are memory locations within the processor itself, storing data and instructions. There are several different registers in the CPU.
   • Immediate Access Store (IAS)
     • This holds instructions and data that is ready to be used by the CPU. It is a set of memory locations. This can also be called main memory, RAM, primary memory.

3. Control Unit

   • The CU manages how instructions are executed, as well as controlling the fetch-decode-execute cycle. It fetches instructions and data from wherever they're stored in the memory unit. Its next important job is to decode an instruction so it knows what needs to be done. Once this is done, it then executes the instruction.
   • Any program or application you write is made up of a sequence of instructions. When you 'run' a program it is the control unit is fetching, the decoding and finally executing every instruction, one after another. This is known as the fetch-decode-execute cycle.

4. Immediate Access Store

   • The IAS is the place where programs and the data that is needed by programs are held, ready to be fetched then decoded and executed by the CPU.
   • The CPU may also use this place to store the results of any processing it does. It can be thought of or made up of lots of individual memory locations, each capable of storing a byte of data.
   • When you open an application, the application is moved from storage on your hard disk into the IAS, ready for the CPU. When you open a file, it is also moved from a storage device into the IAS. When you save work, on the other hand, it's moved from the IAS back into a storage device, such as your hard drive. It's important to remember that the operating system, the programs you're currently using and any data you're currently using are worked on by the CPU from the IAS, not from storage. The CPU does not work directly with storage devices, only the IAS.

5. Registers

   • An integral part of the CPU.
   • A type of memory that can be accessed very quickly.