A computer is based on the von Neumann stored program concept.
Describe how the CPU uses main memory (RAM) during the fetch-decode-execute cycle.
Mark scheme
Award up to two marks for a linked description such as:
- The program's instructions (and the data it uses) are held in main memory/RAM (1) so the CPU fetches both the instructions and the data from that one memory (1)
- The CPU fetches the next instruction from main memory/RAM (1) using the address of the memory location that holds it (1)
- The CPU reads/fetches data from main memory/RAM (1) when an instruction needs it and writes the result back to main memory/RAM (1)
- The program's instructions are held in main memory/RAM while the program runs (1) and the CPU fetches them one at a time/one after another to be decoded and executed (1)
Allow 'memory' for main memory/RAM.
Allow 'gets' for fetches/reads.
For both marks, the development must say what the CPU does with the contents of main memory (fetches/reads/writes), not just that main memory is used.
Do not accept answers about the program being stored in secondary storage, such as on a hard drive, or about it being loaded from there.
Do not accept benefits such as the computer running faster or being easier for the user.
Do not accept naming CPU components (control unit, arithmetic logic unit, registers) or buses on their own, without saying what is stored or fetched.
Do not accept 'the computer stores data' on its own, without a link to the instructions of a program.
Example answers and how they were marked
The program's instructions and the data it uses are both held in the same main memory (RAM) (1), so the CPU can fetch the instructions and the data it needs from that one memory (1).
Why: Linked point (1) that instructions and data are stored together in main memory, with the development (1) that the CPU fetches both from the same memory.
The CPU fetches the instructions from main memory (1) so that they are processed more quickly (1).
Why: The first point earns (1) for the CPU fetching instructions from main memory. The second statement is a benefit (speed), which the 'Do not accept benefits' line rules out, so it earns nothing.
The control unit decodes each instruction (1) and the ALU carries out the calculations (1).
Why: Both statements name CPU components without saying what is stored, fetched or written, which the mark scheme tells examiners not to accept, so neither earns a mark.