3.1.4 - Hardware

3.1.4 - Hardware

1. Central Components of a Computer System

   • Arithmetic Logic Unit (ALU)
     • Performs two sort of operations on data:
       • Arithmetic
         • Addition
         • Subtraction
         • Multiplication
         • Division
       • Logical
         • Comparison
         • Less than?
         • Greater than?
         • Equal to?
       • Acts as a conduit through which all I/O to the computer is done.
         • Gateway to the processor.
   • Control Unit
     • Co-ordinates and controls all the operations carried out by the computer.
     • Operates by repeating three operations:
       • 1. Fetch
         • Causes the next instruction to be fetched from main memory.
       • 2. Decode
         • Produces signals which control other parts of the computer such as the ALU.
       • 3. Execute
         • Causes the next instruction to be executed.
   • Memory Unit (RAM)
     • Quick and easily accessible information can be read from and written to by the Control Unit.
     • Access to and from the Memory Unit is very fast.
     • Normally contains:
       • Operating System (OS)
       • Software currently in use.
       • Data which the software is using.
     • Volatile
       • Data is lost when the computer is shut down.
   • Registers
     • An extremely fast piece of on-chip memory, for temporary storage.
     • The registers you need to know are:
       • Program Counter (PC)
         • Holds the address of the next instruction to be executed.
       • Memory Address Register (MAR)
         • Holds the address in memory of the location containing:
           • The next piece of data to be read.
           • The next instruction to be used.
       • Memory Data Register (MDR)
         • The contents of the address specified in the MAR are copied to the MDR. This may be:
           • An instruction/operation.
           • Data to be used (with an instruction).
           • Data to be copied to an address.
       • Current Instruction Register (CIR)
         • Holds the instruction while it is being decoded/executed.
         • The contents of the MDR are copied to the CIR if it is an instruction.
         • Contains the operator and the operand of the current instruction.
       • Status Register
         • Contains "status" bits or flags that are set or cleared based on the result of an instruction.
       • Accumulator
         • Works with the ALU to performs arithmetic functions.
         • Temporary storage areas.
   • Buses
     • A set of parallel wires connecting two or more components of the computer.
     • There are three buses:
       • Control Bus
         • Carries command and control signals to and from every connected device.
         • Fully bi-directional for all connected devices.
       • Address Bus
         • Communicates the physical addresses of computer memory elements/locations that the requesting device wants to access.
       • Data Bus
         • Transports the actual physical data to and from the memory.
         • Both single-directional and bi-directional.

2. Device Connectivity

   • Hard-wire (Physical)
     • Copper Coaxial
     • Fibre Optic
     • Copper Twisted Cable
   • Wireless (Non-physical)
     • Wifi
     • Satellite
     • Bluetooth
   • Measured in the rate at which data can be transmitted.
     • Baud Rate
   • Data rate can be increased by:
     • Using higher quality cable.
       • Less attenuation.
     • Increasing frequency at which data is sent.
   • Each signal can be sent on a different frequency or wavelength.
     • The number of frequencies that can be used before they interfere with each other is called the bandwidth.
       • Copper has low bandwidth.
       • Wireless has high bandwidth.
       • Fibre optic has very high bandwidth.

3. Memory

   • Main Memory
     • Random Access Memory (RAM)
       • Volatile (Wiped when the computer is turned off).
       • Used to store programs which are currently running.
       • Stores data which is currently being processed.
     • Read Only Memory (ROM)
       • Non-Volatile
       • Contents are permanently etched into the memory chip at the manufacturing stage.
   • Secondary Storage
     • Magnetic
       • Data recorded onto magnetic disks.
       • Mechanical parts move over the disk's surface to read and write data.
       • Advantages
         • Cheap.
         • Large storage capacity.
       • Disadvantages
         • Not portable.
         • Durability can be an issue.
     • Optical
       • Data is recorded onto optical disks.
       • Lasers read and write data.
       • Advantages
         • Highly portable.
         • Cheap.
       • Disadvantages
         • Low storage capacity.
         • Easily damaged.
     • Solid State
       • Data is recorded onto solid state memory chips.
       • Disadvantages
         • Expensive.
         • Less storage capacity than magnetic storage.
       • No mechanical moving parts.
       • Advantages
         • Fast.
         • Durable.

4. Transfer of Data

   • Fetch/Decode/Execute Cycle
   • Interrupts
     • A signal from some device or source seeking the attention of the processor.
     • The interrupt signal is sent along the control line to the processor.
       • The currently executing program is suspended and control is passed to an ISR (Interrupt Service Routine).
     • There is a register in the CPU called the Interrupt Register.
       • It is checked at the beginning of every F/D/E Cycle.
   • Buffers (Cache)
     • Used to help match the speeds of many devices.
     • Used between the high processing speeds of the CPU and the slower speeds of the Main Memory.
     • Data can be written to a buffer as though it were Main Memory.
       • The high speed processor can then carry out the next operation whilst the Main Memory reads from the buffer instead.

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