Basically it fetches the instructions from memory, decodes the instruction then executes it. Calculations are performed in the arithmetic logic unit, the answers are then stored in the accumulator. Copies of commonly used instructions are store in the cache.
- Contents of the PC are sent to the MAR,
- Memory address stored in the MAR is placed on the Address Bus and sent to main memory (RAM),
- Instruction at this address is placed on the Data Bus and sent to the MBR - simultaneously, the contents of the PC are incremented to point to the next instruction to be fetched,
- Contents of the MBR are sent to the CIR - this is to clear the MBR for the next fetch cycle, else the current MBR contents would be overwritten,
- Contents of the CIR are decoded into opcode and operand,
- Opcode and operand are sent to the ALU where they're executed,
- Result of this execution stored in the accumulator (ACC)
I would imagine this explanation is valid for both GCSE and A Level, on any exam board.