Processing Techniques & Memory/Storage/IO Management

This is one of the densest theory subtopics — it covers how a CPU actually executes an instruction, how the OS juggles multiple programs, and how the system copes when RAM isn't enough.

The Machine Cycle is the repeating sequence every single CPU instruction goes through:

Step	What happens
Fetch	The CU retrieves the next instruction from memory (RAM), using the address in a special register
Decode	The CU translates the fetched instruction into signals the rest of the CPU understands
Execute	The ALU (or other CPU part) actually performs the operation
Store	The result is written back to a register or memory

Processing techniques — comparing software and hardware approaches to doing more than one thing at once:

Multi-tasking (software) — the OS rapidly switches the CPU between multiple programs, giving the illusion that they run simultaneously.
Multi-threading (software) — a single program splits its work into multiple threads that can be scheduled and (on multi-core/hyper-threaded hardware) run more efficiently.
Hyper-threading and multi-processing (hardware) — see 11.1.3; the hardware-level ability to genuinely or apparently handle more than one instruction stream.

UEFI (Unified Extensible Firmware Interface): A modern replacement for BIOS — faster boot times, supports larger drives, has a graphical interface, and includes better security (e.g. Secure Boot).
BIOS: The older firmware standard for initialising hardware at start-up before handing control to the OS.
CMOS: A small battery-powered chip that stores BIOS/UEFI settings (like system time and boot order) even when the computer is off.
Interrupt: A signal sent to the CPU (by hardware or software) that temporarily pauses the current task so a more urgent one can be handled immediately.
IRQ (Interrupt Request): A specific line/number a device uses to signal an interrupt to the CPU, so the CPU knows which device needs attention.
IO Range: A block of addresses reserved for a specific device to communicate with the CPU.

Virtual memory — a technique the OS uses when RAM fills up:

Paging — data is divided into fixed-size blocks (pages); when RAM is full, inactive pages are moved to secondary storage to free up space.
Swapping — the process of moving those pages between RAM and a dedicated area of secondary storage (the swap file/page file).
Effect on processing speed: because secondary storage is far slower than RAM, heavy reliance on virtual memory (excessive paging, sometimes called 'thrashing') significantly slows the system down.

Example

You open 15 browser tabs and your 8GB RAM fills up. The OS starts paging the least-recently-used tabs to your SSD's swap file. Switching back to one of those tabs feels sluggish because the OS has to read that data back from the (much slower) SSD into RAM before you can use it again.

💡 Exam Tip

Don't confuse an interrupt with an IRQ: the interrupt is the event/signal itself; the IRQ is the identifying line/number that tells the CPU which device raised it.