The Processor

The Processor

1. Components

   • Control Unit
     • This directs the operation of the processor, tells the ALU, main memory and I/O controllers how to respond to a programs instructions.
     • This supervises the Fetch-Excecute cycle
     • It also makes sure all of the data is routed correctly - is put in the correct register or memory location
   • ALU (Arithmetic Logic Unit)
     • Performs arithmetic and logical operations on data in order to manipulate it
       • Some examples Logical Operations
         • AND
         • OR
         • NOT
       • Some Examples of Arithmetic Operations
         • +
         • -
         • >
     • The ALU is sent an Op-code and the operands (more on this in the machine code section)
   • Clock
     • A device that generates an electrical signal used to synchronise the components of a computer
     • The clock speed is the number of cycles per second that this component produces (more on the other side)
   • Registers
     • General Purpose Register
       • A register with no assigned dedicated function which is available to a programmer to use in programs to temporarily store data
     • CIR (Current Instruction Register)
       • This register stores the instruction that is currently being executed by the processor
     • MAR (Memory Address Register)
       • This stores the memory address of the location where the data is about to be read from or written too
     • MBR (Memory Buffer Register) Sometimes called the MDR (Memory Data Register)
       • This stores data that has just been read from main memory or is about to be written to main memory
     • PC (Program Counter)
       • This stores the address of the memory location of the next instruction that will be fetched by the processor
     • Status Register
       • This keeps track of the states of various parts/functions of the computer, for example if an overflow has occured
     • [Register Name] means contents of the register
     • A small section of temporary storage that is part of the processor. Stores data or control instructions during the Fetch-Excecute cycle

2. Fetch Excecute Cycle

   • Briefly
     • Fetch: The next instruction is fetched from main memory into the control unit in the processor
     • Decode: The control unit decodes the instruction so that the processor knows what the binary code at that address means
     • Execute: The program carries out that instruction (or instruction obayed)
   • In Detail
     • Fetch
       • 1. The PC holds the address of the next instruction
         • 2. This address is sent to the MAR and then along the address bus to the main memory
           • 3. The instruction at this address is sent along the data bus to the MBR
             • 4. This is then transferred to the CIR
               • 5. The PC is increased by 1 (incremented by 1)
                 • Decode
                   • 1. The control unit decodes the instruction which is currently in the CIR
                     • Execute
                       • 1. The program carries out that instruction and we start off at fetch again

3. What is it?

   • The component that carries out computation (arithmetic and logic operations) on data by following instructions. Manipulates data and controls the function of the computer.
   • It handles all the instructions that it receives from the user and from the hardware and software
   • Physically it is made up of a thin slice of silicone approximately 2cm^2
     • By using microscopic techniques , the silicone is implanted with millions of tiny resistors, connected together by microscopic wires called busses

4. The speed

   • Clock Speed
     • Measured in Hz or more commonly GHz
     • The number of instructions the processor can perform per second
     • A 3GHz processor can carry out 3000 million instructions per second
   • Factors affecting the speed
     • Clock Speed
       • You can increase the clock speed of your processor, called overclocking, this will speed up the processor performance
     • Bus width
       • Increasing the width of the data bus means that more bits and therefore more data can be transferred in one pulse of the clock
       • Increasing the width of the address bus wil lincrease the amount of memory that can be addressed and therefore allows more memory to be installed onto the computer
     • Word Length
       • Higher word length means more data can be manipulated in one pulse of the clock.
       • A word is a collection of bits that can be addressed and manipulated as a single unit
       • Computers generally have a word length of 32 or 64bits
     • Multiple Cores
       • Multiple instructions can be executed simultaneously giving a significant increase in performance.
       • This is the idea that having multiple processors will increase performance because more actions can be performed at once
     • Cache Memory
       • Caching is a technique where instructions and data that are needed frequently, are placed into a temporary area of memory that is seperate from main memory.
       • This memory is much faster that main memory so programs can be sent to the processor more quickly and therefore run faster

5. Architecture

   • Von Neumann
     • Stored Program Concept
       • The concept that program instructions and data are stored together in the same memory
     • Data and instructions are stored in the same memory and are accessed by the same buses. Often used in general purpose computing.
   • Harvard
     • Data and instructions are stored in different parts of memory and are accessed by separate buses. Often used in embedded systems.
       • Embedded Systems include: Mobile Phones, Burglar alarms, etc.
     • Can be executed faster and more efficiently as the same bus isn't used