Computer Science OCR A Level Practice Questions and Answers

Start LDA one OUT LDA zero OUT LDA count SUB one STA count BRP start HLT One DAT 1 Zero DAT 0 Count DAT 3 Explain, giving an example, how pipelining in a CPU could speed up the execution of this program. ( 3 marks ) An instruction can be fetched as the previous one is being decoded and the one before that is being executed.E.g. LDA zero can be fetched, while OUT is being decoded and LDA start one is being executed. Describe one issue the line BRP start may cause for a CPU using pipelining. (2 marks) BRP could be followed by one of two possible instructions, which one will only be determined at execution .Meaning the wrong one may be fetched / decoded. Pipelining is one factor that affects the performance of a CPU. Identify one other factor. ( 1 mark) clock speed Name and describe three buses used to convey information between the special registers. ( 6 marks) Control bus : transmits control signals from the control unit to other parts of the processor. Data bus : carries the data from one place to another. Address bus : carries the location address where the data is going to or from. What does the Program Counter (PC) do? ( 2 marks) Is needed to store the address of the next instruction to be processed. Value is then sent to the MAR .After sending the value the PC is incremented and changed to address held in CIR if the operation is a Jump. What does the memory address register do ( MAR )? ( 2 marks) Contains the address of the instruction to be accessed in memory , address of instruction sent from PC .Contains the address of the data to be accessed in memory , address of data sent from CIR. What does the memory data register do ( MDR) (2 marks) Contains the instruction which has been accessed from memory . Contains the data which has been accessed from memory . That is referenced by the MAR . Acts as a buffer Describe two ways in which the accumulator is used? ( 4 marks ) Temporary storage for data being processed during calculations in the arithmetic logic unit. Used as a buffer . Stored data and control info. Describe the role of the control unit in the processor. ( 2 marks) Decodes instructions. Sends control signals to coordinate movement of data through the processor / execute instruction. .Controls buses