Introduction — what this is and why it matters
A computer is useless if it cannot talk to the outside world. I/O (Input/Output) organization is the part of computer architecture that governs how the CPU and main memory exchange data with peripheral devices — keyboards, disks, printers, network cards, sensors. The central problem is a speed mismatch: a CPU executes billions of operations per second, while a keyboard or disk is millions of times slower. I/O organization is the set of techniques that let a fast CPU cooperate with slow, diverse, autonomous devices without wasting its time.
Interrupts are the most important of these techniques. An interrupt is a hardware signal that lets a device say "I need attention now," so the CPU does not have to keep asking. This is the exam favourite: nearly every BCS / Bank IT / IT-officer paper asks something about interrupts, DMA, polling, or memory-mapped I/O. The concepts are also conceptual gold — one clear understanding answers dozens of MCQ variations.
Learning objectives
- Explain the role of an I/O module (device controller) and why it sits between the CPU and a device.
- Distinguish the three data-transfer techniques: programmed I/O, interrupt-driven I/O, and DMA (Direct Memory Access).
- Compare memory-mapped I/O and isolated (I/O-mapped / port-mapped) I/O.
- Define an interrupt, classify interrupt types, and trace the full interrupt cycle step by step.
- Handle multiple interrupts using priority, masking, polling, daisy-chaining, and vectored interrupts.
- Explain DMA operation, cycle stealing vs burst mode, and compute transfer times.
- Solve numerical problems on I/O timing, interrupt overhead, and DMA throughput.