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Edexcel GCSE CS · 3.1

Hardware and the Processor

Section A: 10 multiple choice questions, 10 marks. Section B: 4 written questions, 12 marks. Total 22 marks.

Name

Class

Date

Mark  / 22

Section A

Circle the letter of the correct answer. One mark each.

  1. 1Which register holds the address of the next instruction to be fetched?1
    1. AMAR
    2. BProgram counter
    3. CAccumulator
    4. DMDR
  2. 2What is held in the MDR?1
    1. AThe address being accessed
    2. BThe result of a calculation
    3. CThe number of cores
    4. DData or an instruction moving to or from memory
  3. 3What is the defining feature of the von Neumann architecture?1
    1. AInstructions and data are held in the same memory
    2. BIt has more than one core
    3. CIt uses four registers
    4. DIt stores programs on an optical disc
  4. 4Which component performs comparisons such as testing whether one value is greater than another?1
    1. AThe control unit
    2. BCache
    3. CThe arithmetic logic unit
    4. DThe program counter
  5. 5How does a larger cache improve performance?1
    1. AMore of what the processor needs is close by, so it waits for RAM less often
    2. BIt reduces the number of instructions needed
    3. CIt adds another core
    4. DIt increases the clock speed
  6. 6Why might a quad core processor not be twice as fast as a dual core one?1
    1. AQuad core processors have a lower clock speed by design
    2. BThe cores run at half speed each
    3. CSome tasks are sequential and much software cannot use multiple cores
    4. DCache is shared and therefore useless
  7. 7At what point is the program counter incremented?1
    1. AOnce per program
    2. BOnly after a jump instruction
    3. CDuring the fetch stage
    4. DAt the end of the execute stage
  8. 8What is an embedded system?1
    1. AA computer that cannot be networked
    2. BA computer with only one core
    3. CAny computer without a screen
    4. DA computer built into a larger device to perform one dedicated task
  9. 9What travels along the address bus?1
    1. AThe result of a calculation
    2. BThe instruction itself
    3. CThe clock pulse
    4. DThe memory location being read from or written to
  10. 10Why is the von Neumann bottleneck a limitation?1
    1. AOnly one program can be stored at a time
    2. BInstructions and data share one route to memory, so they compete and the CPU waits
    3. CMemory cannot be written to
    4. DRegisters can hold only one value

Section B

Answer in the space provided. The number of marks tells you how many separate points to make.

  1. 1StateState the purpose of the accumulator.1
  2. 2DescribeDescribe the fetch stage of the fetch decode execute cycle.4
  3. 3ExplainExplain how increasing the size of the cache can improve the performance of a computer.3
  4. 4ExplainA manufacturer is designing a controller for a microwave oven. Explain why an embedded system is more suitable than a general purpose computer.4

Mark scheme

Section A

1B2D3A4C5A6C7C8D9D10B

Section B

1 State the purpose of the accumulator. 1

One mark for each of the following, to a maximum of 1.

  • It holds the result of the most recent calculation

Model answer. The accumulator is the register that holds the result of the most recent calculation carried out by the arithmetic logic unit.

2 Describe the fetch stage of the fetch decode execute cycle. 4

One mark for each of the following, to a maximum of 4.

  • The address in the program counter is copied into the MAR
  • The program counter is incremented
  • The address is sent out on the address bus
  • The instruction is returned on the data bus into the MDR

Model answer. The address held in the program counter is copied into the memory address register. The program counter is then incremented so that it already points to the following instruction. The address in the MAR travels out along the address bus to main memory, and the instruction stored at that address is returned along the data bus into the memory data register, ready to be decoded by the control unit.

3 Explain how increasing the size of the cache can improve the performance of a computer. 3

One mark for each of the following, to a maximum of 3.

  • Cache holds recently and frequently used data and instructions close to the CPU
  • Data in cache is accessed far faster than data in main memory
  • A larger cache means fewer fetches from RAM, so the CPU spends less time waiting

Model answer. Cache is small, very fast memory located inside or extremely close to the processor, holding copies of the instructions and data that have been used recently or are likely to be needed next. Reading from cache takes a small fraction of the time that reading from main memory takes. Making the cache larger means a greater proportion of what the processor asks for is already there, so it has to fetch from RAM less often. Since a fetch from RAM leaves the processor idle while it waits, reducing how often that happens means more instructions are completed each second, which is what performance actually measures.

4 A manufacturer is designing a controller for a microwave oven. Explain why an embedded system is more suitable than a general purpose computer. 4

One mark for each of the following, to a maximum of 4.

  • The controller carries out one fixed, dedicated task
  • It does not need to run other software or a general purpose operating system
  • So a modest processor and very little memory are sufficient, which reduces cost
  • It is also more reliable and uses less power, which matters for an appliance

Model answer. The controller in a microwave has exactly one job, which is to run the cooking programme, drive the turntable and magnetron and read the keypad. Because that job is fixed and will never change, the system does not need to run arbitrary software and does not need a general purpose operating system to manage whatever a user might install. The single program can be written to fit the hardware precisely, so a very modest processor and a few kilobytes of memory are enough, which makes the component far cheaper to manufacture at the volumes involved. It is also more reliable, since there is very little that can go wrong and no software to become corrupted, and it consumes far less power, which matters in an appliance that is connected to the mains continuously.