Computer Systems

Computer Systems

1. CPU (Central Processing Unit)

   • The CPU can only carryout instructions that have been given to it. This can cause some misunderstanding when it is referred to as the 'brain' of the computer because the CPU cannot think. It is limited to the information supplied to it.
   • A Processor can also contain billions of transistors. Transistors are small electronic switches that combine together to build the logic gates. These can then be used to build the circuitry inside a processor.
     • Intel is one of the worlds leading processor companies. In 1965 the founder of Intel, Gordon Moore, predicted that the number of transistors that a processor would be able to use would generally double ever two years. His predictions has been correct for ever two year iteration, but computer scientists have started to realise that we may be reaching the physical limits. However doubling the number of transistors, into the same space, will increase the speed of the the processor. Processing speed has continued to increase exponentially, so we now have smartphone that are more powerful than the desktop computers from as recent as 10 years ago.
       • Processors work at incredible speeds. The processors runs on time with the clock signal. The speed of this signal is measured in hertz
         • 1 Hertz
           • 1 Pulse Per Second
         • 1 Kilohertz
           • 1000 Pulses Per Second
         • 1 Megahertz
           • 1,000,000 Pulses Per Second
         • 1 Gigahertz
           • 1,000,000,000 Pulses Per Second
         • Modern desktops will usually run in the oder of a Gigahertz clock signal (clock speed). A computer with a 4Ghz processor will be capable of carrying out up to 4,000,000,000 instructions per second. Clock speed can be an accurate way of comparing two different processors. However it is possible for a processor with a lower clock speed to out perform a processor that has a higher clock speed. This is because the processors performance can be notably effected by other factors like: cache size, number of cores and pipelining.
           • Processors work by following the fetch-decode-execute cycle. The processor will continually fetch instructions from the memory to decode and execute them.

2. Inside the Processor

   • Registers
     • Registers are memory locations that are in the processor itself, they are designed to work at extremely fast speeds so they can be used by the processor without causing a bottleneck. (A bottleneck is the slowest part of the system that can limit its speed as a whole).
       • PC (Program Counter)
         • The PC job is to know the memory location of the line of machine code that is being executed. The line of code is discretely increased in size to point to the next instructions. In each fech-execute-decode cycle allowing the program to be executed in sequence one by one
       • MDR (Memory Data Register)
         • The MDR stores the data that has been fetched from or stored in memory
       • MAR (Memory Address Register)
         • The MAR's job is to store the address of the data or instructions that are to be fetched from or sent to a location
       • CIR (Current Instruction Register
         • The CIR store the data from the recently fetched instructions that are waiting to be decoded and executed
       • ACC (Accumulator)
         • The ACC stores the results from the calculations made by the ALU (Arithmetic logic unit)
       • General Purpose Register
         • Processors can also contain a General Purpose Register. They are used to temporarily store data being used rather than the slower alternative of sending the data to and from.
       • Buses
         • Buses are the communication channels used by computers to send data around the system. The USB (Universal Serial Bus) is used to transport data between computers and external devices. During the fetch-decode-execute cycle there are three buses that the computer uses: the data bus, control bus and the address bus. The data bus transports data between the processor and the memory, the address bus transports the address of the memory location being read from or written to and the control bus continually send control signals form the control unit.
       • ALU (Arithmetic Logic Unit)
         • The ALU performs the calculations and logical decisions. The result of these calculations will be stored in the accumulator.
       • CU (Control unit)
         • The control unit send out the control signals to co-ordinate how the processor works. It will control how the data moves around certain parts of the CPU and how it will move between the CPU and the memory. Instructions will also be decoded in the CU