1.1. Systems Architecture

• What does CPU stand for?


  Central processing unit.
• What is the CPU for?


  Its purpose is to process data - it fetches data and instructions, decodes them, and executes them.
• What does the data bus do?


  Carries data to and from the memory.
• What does the address bus do?


  Carries addresses stating where you can find data in the memory.
• What does the control bus do?


  Carries control signals from the CU.
• What does fetch mean?


  To collect data or instructions from the RAM.
• What does decode mean?


  To work out how to carry out an instruction.
• What does execute mean?


  To carry out an instruction.
• What is data in terms of systems architecture?


  Letters and numbers used by instructions.
• What is an opcode?


  The instruction - e.g. ADD
• What is an operand?


  The data the instruction needs.
• What are the parts of the CPU?

  • Control unit (CU)
  • Arithmetic logic unit (ALU)
  • Cache
• What does the CU do?


  Decodes instructions, and sends signals to tell other parts of the processor what to do - it controls the flow of data.
• What does the ALU do?


  Carries out calculations and makes logical decisions.
• What is the cache?


  Very small, and very fast memory that holds only the most commonly used instructions for fast access.
• What are the kinds of registers there are?


  • Program counter (PC)
  • Memory address register (MAR)
  • Memory data register (MDR)
  • Current instruction register (CIR)
  • Accumulator
• What is a register?

  Small fast piece of memory inside the CPU that holds items the CPU is using.
• What does the PC, program counter do?


  Stores data for the next instruction - it is incremented in each FDE cycle.
• What does the MAR do?


  Stores memory addresses to show where data will be fetched from or stored to next.
• What does the MDR do?


  Stores data fetched from memory or data that will be sent to memory.
• What does the CIR do?


  Holds an instruction whilst being decoded.
• What does the accumulator do?


  Holds the results of processing from the ALU.
• What are the factors affecting the CPU?


  • Clock speed
  • Number of cores
  • Cache size
• How can the clock speed affect the CPU?


  It is a measure of how many instructions can be carried out per second in hertz (Hz) - therefore higher clock speed means more instructions per second.
• How can the number of cores affect the CPU?


  They are independent processing units that can work on one instruction at a time each - more cores mean that there will be more instructions done in a given amount of time.
• How can cache size affect the CPU?


  They are faster to access than RAM, so commonly used instructions held there are accessed more quickly - more cache means that more instructions can be accessed quickly, and therefore faster processing.
• What are embedded systems?


  Systems with limited functions that are hard to or cannot be changed - they have dedicated, specialist processors designed for the function.
• Where may embedded systems be used?


  Appliances such as washing machines, dishwashers, microwaves or simple computing devices such as aircons.
• Describe the fetch process of the FDE cycle:


  1. The PC contains the address of the next instruction and passes it to the MAR
  2. The MAR fetches data from that address in the memory and it is sent to the MDR
  3. The MDR holds the loaded instruction before sending it to the CIR
• Describe the decode process of the FDE cycle:


  1. The CIR holds the instruction that is being decoded
• Describe the execute process of the FDE cycle:


  1. The ACC holds the results of processing.