Computer Systems & Architecture is a core part of A-Level Computer Science. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
CPUProcesses instructions via the fetch–decode–execute cycle.
Von Neumann architectureStores data and instructions in the same memory.
RAM & ROMRAM is volatile working memory; ROM is permanent.
CacheFast memory close to the CPU for frequently used data.
RegistersTiny, fast stores inside the CPU (e.g. the accumulator).
Clock speedHow many cycles the CPU performs per second.
Program counter (PC)The register holding the memory address of the next instruction; it is incremented during fetch, or overwritten by jumps and branches.
Memory address register (MAR)The register that holds the address of the memory location about to be read from or written to.
Memory data register (MDR)The register that temporarily holds data or an instruction transferred between the processor and main memory.
AccumulatorA general-purpose register in which the results of calculations performed by the arithmetic logic unit are held.
System busesThe address bus carries memory addresses, the data bus carries the data itself, and the control bus carries control and timing signals.
Virtual MemoryAn abstraction giving each process its own large address space.
PageA fixed-size block of virtual memory, often 4 KB.
Page TableData structure mapping virtual pages to physical frames.
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
What does the CPU do?✗ Permanently stores all the computer's files. ✓ Processes instructions (fetch–decode–execute).
How do RAM and storage differ?✗ They are the same thing. ✓ RAM is fast, volatile working memory; storage keeps data without power.
What is system software like an operating system for?✗ Doing the user's tasks like writing essays. ✓ Managing hardware and running other software.
Does the program counter hold the instruction currently being executed?✗ Yes - the program counter contains the instruction being run. ✓ No - the PC holds the ADDRESS of the next instruction to fetch; the fetched instruction itself is held in the current instruction register.
Does the accumulator store memory addresses?✗ Yes - the accumulator accumulates the addresses of used memory. ✓ No - the accumulator holds data, specifically the results of ALU operations; addresses are held in the PC and MAR.
Practise Computer Systems & Architecture — free games
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