Comp sci

• Control unit


  • Coordinates all activities of the CPU
  • Directs flow of data between the CPU and other devices
  • Accepts the next instruction, decodes it, handles its execution, and stores the resulting data back in memory or registers
  • Sends memory read and write requests to main memory on the control bus, as well as other command and control signals such as bus requests, bus grants, etc.
  • Makes extensive use of the status register and clock
  • Coordinates and communicates with all parts of the CPU
• Program counter


  Holds the address of the next instruction to be executed
• Memory address register


  • Holds the address of the memory location from which data or an instruction is to be fetched or to which data is to be written
  • Sends these addresses to memory down the address bus
• Memory data register


  • Used to temporarily store the data which is read from or written to memory
  • Sometimes known as the memory buffer register (MBR) and is often nicknamed the "gateway to the processor"
  • All data to and from memory must pass down the data bus and must pass through the MDR
• Current instruction register


  • Holds the current instruction being executed
  • The contents of the MDR are copied to the CIR if it is a new instruction
  • Contains the opcode and operand of the current instruction
• Arithmetic logic unit


  • Performs arithmetic and logical operations on data
  • Arithmetic operations in fixed and floating point numbers: ADD, SUBTRACT, MULTIPLY, DIVIDE
  • Bitwise operations: Shift left and right
  • Boolean operations: Complement, AND, OR, NOT, XOR
  • Stores the results of computations in general-purpose registers
• System software is software that a computer must have to function. Without system software, a computer would be very difficult to work, or wouldn't work at all.
• Operating system


  The most important piece of software in a computer. Without an operating system, a computer would be an expensive radiator.
• What the operating system provides

  • User interface
  • Memory management
  • Hardware/peripheral management
  • CPU usage management/interrupt handling
  • Security
  • Ability to run other software
  • System maintenance utilities
• User interface


  The operating system controls the hardware and provides an interface for the user
• Multi-tasking


  1. Dividing one CPU's cycles between all the processes

  2. Each process gets a few CPU cycles to complete a few instructions and then is swapped out

  3. Gives the impression that everything is being done at once
• Multi-processing


  1. Having more than one CPU core to work with

  2. Work can now be truly divided out and processes can be executed simultaneously
• Memory management


  • Allocates memory to a program
  • Reclaims memory when it is not in use
  • Swaps memory blocks in and out of virtual memory
  • Prevents programs from accessing each other's memory
• Accumulator


  • One of a number of general-purpose registers that modern CPUs have
  • Data or control information is often stored in them
  • The results of calculations carried out by the ALU can be temporarily stored here
  • The more general-purpose registers a processor has, the faster it may operate
• Address bus


  Carries memory addresses that identify where data is being read from or written to
• Data bus


  Carries the binary bits and bytes that make up the actual information being transmitted around the CPU/computer
• Control bus


  Carries command and control signals to and from every other component of the CPU/computer