Structure and role of the processor and its components exam tips

Exam tips

• Use precise register descriptions

  State both what a register stores and when it is used.

  For example, saying that the program counter stores the address of the next instruction distinguishes it from the current instruction register, which stores the instruction currently being processed.

• Describe register transfers precisely

  When explaining the fetch stage, state what is stored in each register and describe the order in which the address and instruction move.

  Precise register roles and correct sequencing demonstrate how the Fetch-Execute cycle operates rather than merely naming its stages.

• Show the power of two

  When calculating the number of possible opcode patterns, write 2 raised to the number of opcode bits before giving the result.

  This demonstrates why 4 opcode bits produce 2^4 = 16 possible patterns and helps secure calculation marks.

• Use precise operand language

  When comparing the modes, explicitly write 'operand is the datum' for immediate addressing and 'operand is the address of the datum' for direct addressing.

  These exact distinctions demonstrate the required understanding and avoid vague statements such as 'immediate is faster' or 'direct uses memory,' which do not define the modes.

• Trace instruction effects in order

  For an instruction sequence, record the accumulator and any changed memory locations after every LOAD, arithmetic operation and STORE.

  A clear trace prevents errors caused by forgetting that STORE changes memory, while LOAD and arithmetic operations change the accumulator.

• Link saving to resuming

  When explaining the volatile environment, explicitly state that it is saved so the interrupted program can continue correctly after the ISR.

  This shows both what is saved and why it is necessary, rather than merely repeating that the environment is saved.

• Link each factor to its specific effect

  For every performance factor, state what it changes and then explain how that change can improve processing.

  Explanations receive more credit than unsupported statements such as saying that a component simply makes the processor faster.