This section covers computer architecture — the CPU and its components, how instructions are processed, types of memory and storage, peripheral devices, and the role of system software.
CPU components: ALU (arithmetic & logic operations), Control Unit (fetches and decodes instructions, controls data flow), registers (small fast memory). Key registers: Program Counter (address of next instruction), Memory Address Register (address being accessed), Memory Data Register (data being transferred), Accumulator (stores results). Clock speed (Hz), number of cores, and cache size affect performance. Fetch-Decode-Execute cycle: fetch instruction from RAM → decode instruction → execute it → repeat.
Primary memory: RAM (volatile, read/write, stores running programs) and ROM (non-volatile, read-only, stores BIOS/boot instructions). Cache: small, fast memory between CPU and RAM. Secondary storage: non-volatile, larger capacity. Types: magnetic (HDD — high capacity, moving parts), solid-state (SSD — fast, no moving parts, more expensive), optical (CD/DVD/Blu-ray). Virtual memory: uses hard disk as extension of RAM when RAM is full (slower). Cloud storage: remote servers accessed via internet.
System software: manages hardware. Operating system: manages files, memory, peripherals, user interface, security, multitasking. Utility software: antivirus, disk defragmenter, backup, compression. Application software: performs user tasks (word processor, spreadsheet, browser). Interrupts: signals that cause CPU to pause current task and handle the interrupt (e.g., keyboard press, printer ready, error). Interrupt service routine (ISR) handles each interrupt, then returns to original task.
When RAM is full, the operating system uses virtual memory — it transfers less-used data from RAM to a section of the hard disk (called a swap file or page file). When that data is needed again, it is swapped back into RAM and other data is moved to disk. This allows the computer to run more programs than physical RAM would permit, but it is much slower than using RAM because hard disk access is thousands of times slower. Excessive use of virtual memory causes "thrashing" — the system spends more time swapping data than executing programs, making performance very poor.
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