1.1.1 The structure and function of the processor

The structure and function of the processor

• ALU (Arithmetic Logic Unit):
  • Carries out calculations (e.g. adding, subtracting, multiplying & dividing, logical operations and comparisons; is A greater than B)
  • The result is usually stored in the accumulator
• Control Unit:
  • Controls the way data moves around the CPU
  • Controls and monitors the flow of data between the CPU and other components e.g. input devices, memory, graphics card etc.
  • Decodes/Executes the instructions provided by the program
• PC (Program Counter):
  • Holds the address in memory of the next instruction to fetch
  • Used to service an interrupt routine
  • Software that is currently running controls the PC
  • The PC passes the next address to the MAR as part of the standard fetch-execute cycle
• Accumulator:
  • To collect the result of the currently running instruction
  • To store intermediate results of the current data processing
  • Holds data whilst calculations are performed
  • The final results are then either stored in another register or a memory location
  • Without the accumulator data would be stored in RAM -> much slower
• MAR (Memory Address Register):
  • Holds the address of the next piece of data or program to be fetched or stored
  • The address of the data or instruction being accessed is temporarily stored in the MAR. This address is passed to RAM by means of the address bus
  • The PC copies the address of the next instruction into here
• MDR (Memory Data Register):
  • When the data or program is fetched it is temporarily held in the MDR
  • Data (unlike instructions) goes both ways; data can be fetched from main memory and brought to the CPU to be processed. After processing it may be sent back to RAM to be stored (short term). All data must pass throughout the MDR whether it's on its way to the CPU or RAM.
  • All data and instructions pass via the data bus
• CIR (Current Instruction Register):
  • Holds the current instruction, to be executed, having been fetched from memory
• Control Bus:
  • Receives and sends control information between other parts of the computer
  • Sends control signals from the control unit to coordinate how the processor works
• Data Bus:
  • Handles both data and instructions
  • Allows data to pass from area to another, e.g. shifting data between the CPU and main memory
  • The width of the bus determines how many bits the CPU can handle at once
  • Typical widths are 8-bit, 16-bit, 32-bit and 64-bit
• Address Bus:
  • Carries the address of the memory location being read from or written to
  • The CPU places the address of the data or instruction on the address bus
  • The width of the…