COMPUTER ORGANISATION + ARCHITECTURE

• Processor


  Executes program instructions in order to run applications. Contain an ALU, control unit and registers.
• Main memory


  Random access memory (RAM) and read only memory (ROM) used to store program instructions and frequently used data. Much faster than secondary storage so storing data here helps processor execute instructions quickly
• Bus


  Series of parallel wires connecting internal components to allow signals to pass between them.
• Width


  Number of parallel wires in a bus, has direct relationship to the number of bits that can be transferred simultaneously by the bus.
• Address bus


  Used to transport memory addresses specifying where data should be sent to/fetched from. Increasing width increases range of addresses it can specify, increasing computers amount of addressable memory. Number of addressable memory locations = 2^n if n is number of parallel wires. Unidirectional.
• Addressable memory location


  Portion of memory that can be accessed by its address. If there are not enough available, some portions may go unused
• Data bus


  Sends data and instructions. Increasing width increases volume of data that can be transferred at any one time. Bidirectional.
• Control bus


  Carries control signals that regulate operation of the computer system and carries the clock signal. Bidirectional.
• I/O controllers


  Hardware that control the communication of data between processor and external hardware devices
• Von Neumann architecture


  Instructions and data are stored together in the same memory and share buses for fetching both instructions and data, causing systems to perform worse due to bottlenecking and data can be processed as instructions vice versa. Used in general purpose computers e.g. laptops and smartphones
• Harvard architecture


  Processor uses 2 separate memory locations for instructions and data, reduces bottlenecking and limits errors. Allows different characteristics e.g. memory used for instructions may be read only so they cannot be altered. Used in embedded systems e.g. digital signal processing
• Stored program concept


  Serially fetching and executing machine code instructions stored in main memory by a processor that performs arithmetic and logical operations
• Stored program concept allows computer to switch out one set of instructions for another stored in main memory to run multiple different applications.
• Arithmetic logic unit (ALU)


  Performs arithmetic and logic operations e.g. addition, AND, OR
• Control unit


  Responsible for controlling components of processor and FDE cycle
• Registers


  Small storage locations used to hold data temporarily. Have high read and write speeds.
• General purpose registers


  Can be used as storage for any data that is required by instructions during execution.
• Special purpose registers


  Assigned to the storage of specific information and include PC, CIR, MAR, MBR, SR.
• Program counter (PC)


  Holds memory address of the next instruction to be executed in FDE cycle
• Current instruction register (CIR)


  Holds instruction currently being executed by processor
• Memory address register (MAR)


  Holds memory address of the memory location being read from / written to
• Memory buffer register (MBR) / memory data register (MDR)


  Holds contents of the memory location being read from / written to
• Status register (SR)


  Holds number of bits, values which can change to indicate occurrence of interrupt
• Processor instruction set


  Group of instructions it can carry out. Each processor has its own set so instructions for one may not be compatible with others. Stored in machine code and consist of opcode and one or more operands.
• Opcode


  Specifies the type of operation to be carried out
• Operands


  The pieces of data on which the operation is performed
• Immediate addressing


  The actual value
• Direct addressing


  The memory address of the value
• Hardware interrupt


  E.g. I/O controller informing processor that mouse has been moved/keyboard key has been pressed
• Software interrupt


  E.g. unexpected error like divide by 0 or stack overflow
• Interrupts are detected as changes in the content of the SR between the fetch and execute stages of the FDE cycle
• Vectored interrupt method


  Way of handling interrupts. When interrupt occurs, processor stops executing current program and places register contents onto system stack to save the volatile environment. Then loads interrupt service routine- instructions for handling specific interrupt. When fixed it restores volatile environment and resumes execution.
• Factors affecting processor performance

  • Number of cores
  • Cache memory
  • Clock speed
  • Word length
  • Address bus width
  • Data bus width