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4 questions · 11 marks
Computers manipulate binary patterns.
Convert the binary pattern 1010 0100 to denary.
Award one mark for:
164
Why: Correct: 128 + 32 + 4 = 164 (1).
37
Why: Reads the place values from the wrong end (the left-hand bit as 1), so the value is wrong.
3
Why: Counts the 1s instead of adding their place values.
Images and sound can be stored using lossy compression.
Define the term lossy compression.
Award one mark for any of the following up to a maximum of two marks:
Allow 'some data is deleted/discarded and cannot be recovered' for the second mark.
Do not award the first mark where the response also states that no data is lost.
Do not accept 'removes repeated data' (this describes lossless compression).
Lossy compression reduces the size of a file by permanently removing some data, so the original cannot be restored exactly.
Why: Reduces the size of a file (1); permanently removes some data (1); maximum two marks awarded.
It makes a file smaller.
Why: Reduces the size of a file (1); no reference to data being permanently lost.
It makes a file smaller without losing any of the data.
Why: States that no data is lost, which describes lossless compression; the first mark is not awarded.
A computer can be used by more than one person.
Describe how an operating system uses access rights to protect each user's files.
Award up to two marks for a linked description such as:
Do not accept responses about asking for a password/username, as this checks who the user is, not what they are allowed to do.
Do not accept 'files are protected/safe' or 'access rights protect files' without saying how access is limited (e.g. read-only access, permissions for different users).
A user can be given read-only access to a file, so they can open it but cannot edit or delete it.
Why: Read-only/read access point (1) and linked development that the user cannot edit/delete the file (1).
A user can be given read-only access to a file.
Why: Read-only/read access point (1); no development saying what this prevents, such as being unable to edit/delete the file (0).
The operating system asks for the password again before the file opens.
Why: Describes asking for a password, which checks who the user is, not access rights to the file; no creditworthy point.
A college is buying a new set of computers for its computer science laboratory. A technician explains to the students that the CPU in each computer repeatedly carries out the fetch-decode-execute cycle while a program is running.
Discuss the roles of the control unit, arithmetic logic unit (ALU) and registers in the fetch-decode-execute cycle.
Your answer should include:
Indicative content
Control unit and ALU
Registers
Working together as the cycle repeats
Levels
Additional guidance
The control unit controls the other parts of the CPU in each of the college's new computers. In the fetch stage, the address of the next instruction is copied from the program counter into the memory address register and sent to main memory over the address bus. The instruction comes back over the data bus and is held in the memory data register. The control unit then decodes the instruction, working out which operation has to be carried out. In the execute stage it sends control signals to the ALU or to main memory so that the instruction is carried out. The ALU performs arithmetic operations, such as adding two values, and logical operations, such as comparing two values, on data held in registers, and the result is stored in a register such as the accumulator. Registers are small, very fast memory locations inside the CPU, so the CPU holds the instruction and the data it is working on without going back to slower RAM every time. The program counter is then incremented so that it holds the address of the next instruction, and the cycle repeats. The clock sends a regular pulse so that the stages of the cycle happen in step.
Why: Comprehensive understanding of the control unit, ALU and registers, with each component linked to the fetch, decode or execute stage and applied to the college's new computers, and sustained lines of reasoning through the cycle. Level 3 (5–6): all three bullets covered and linked, so 6.
The control unit is part of the CPU in the college's new computers. When an instruction is fetched, the control unit decodes it to find out what has to be done, and later it signals the other components to carry out the instruction. The ALU does arithmetic and logical operations, so it is used in the execute stage when a calculation is needed. Registers are fast memory locations inside the CPU that hold the data being worked on. For example, the program counter holds the address of the next instruction, so the CPU knows where to fetch from next.
Why: Adequate understanding: the control unit and ALU are linked to the decode and execute stages and to the college's computers, and the program counter is linked to the fetch stage, so some linkages and lines of reasoning are present. The role of registers is thin and there is no sequence through the whole cycle, so it does not reach Level 3. Level 2: 3 marks.
The control unit is the brain of the CPU and makes all of the decisions. The ALU is used to do arithmetic, for example adding numbers together. Registers are like RAM but they are inside the CPU, and they store the data a program needs. The fetch-decode-execute cycle fetches, decodes and then executes an instruction.
Why: One basic, independent point: the ALU carries out arithmetic operations (1). 'The control unit is the brain' and 'registers are like RAM' are the named misconceptions and gain no credit, and restating the three stage names gives no role. Level 1: 1 mark.
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