Computer Systems: Hardware, Software and System Architecture
This domain tests the hardware and architecture strand: what each component does, how an instruction moves through the processor, and how system software differs from application software. Expect definitions, labelled descriptions and short explanations rather than long essays.
Core architecture
- The CPU fetches an instruction from memory, decodes it, executes it and stores the result. The program counter (PC), instruction register (IR), arithmetic logic unit (ALU) and control unit (CU) support this cycle.
- Von Neumann architecture stores instructions and data in the same addressable memory and transfers them over shared address, data and control buses. Harvard architecture separates instruction and data memory.
- RAM is volatile working memory; ROM is non-volatile firmware storage; cache is small, fast memory close to the CPU that reduces average access time. Secondary storage such as HDD, SSD and optical media is non-volatile and slower.
- Peripherals connect through controllers and ports; device drivers translate operating-system commands into device-specific signals.
Software
- System software manages the machine: the operating system handles process, memory, file and device management, alongside utilities and translators.
- Application software performs user tasks such as word processing, browsing and spreadsheets.
- A compiler translates the whole source program before execution; an interpreter translates and executes line by line; an assembler translates assembly language.
Worked example
Question: state one effect of increasing cache size on performance. Reasoning: cache holds recently used instructions and data; a larger cache raises the hit rate, so the CPU waits less often for slower RAM. Answer: fewer main-memory accesses, so the fetch-decode-execute cycle completes faster.
Traps
Do not say RAM stores files permanently; that is secondary storage. Do not confuse the ALU, which performs arithmetic and logic, with the control unit, which coordinates the cycle. "More RAM always makes a computer faster" is too absolute: extra RAM helps multitasking but does not raise clock speed.