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optional · CSS study notes

I/O Types

CSS Optional — Paper-II, Computer Organization & Architecture

1. Definition

An interrupt is a signal that pauses the CPU's normal instruction sequence to service an event, after which the CPU (usually) resumes exactly where it left off. Interrupts are classified along several independent dimensions.

2. Classification — By Maskability

  • Maskable interrupts — can be selectively disabled or ignored by the CPU via an interrupt-enable flag.
  • Non-maskable interrupts (NMI) — always serviced regardless of CPU state; reserved for critical events (e.g., power failure, hardware fault).

3. Classification — By Vectoring

  • Vectored interrupts — the device directly supplies the address of its Interrupt Service Routine (ISR).
  • Non-vectored interrupts — the CPU must poll to determine the interrupt source.

4. Classification — By Priority Arbitration

  • Daisy chaining — a hardware priority chain; the first requesting device in the chain wins.
  • Polling — the CPU queries each device in turn; priority is software-defined.
  • Independent request lines — each device has its own dedicated request line.

5. Classification — By Origin

  • Hardware interrupts — originate from external devices.
  • Software interrupts — triggered explicitly by an instruction (e.g., system calls, traps).

🗺️ Architecture Diagram — Interrupt Classification

graph TD
    INT["Interrupts"] --> MASK["By Maskability"]
    MASK --> M1["Maskable"]
    MASK --> M2["Non-Maskable - NMI"]
    INT --> VEC["By Vectoring"]
    VEC --> V1["Vectored"]
    VEC --> V2["Non-Vectored"]
    INT --> ORI["By Origin"]
    ORI --> O1["Hardware"]
    ORI --> O2["Software"]

🎯 CSS Exam Angle

Interrupt classification is a common MCQ-trap area and appears as a short question distinguishing maskable/non-maskable or vectored/non-vectored types.

🔁 Repeated CSS Questions

  1. Differentiate between maskable and non-maskable interrupts.
  2. Explain vectored vs non-vectored interrupt handling.
  3. What is daisy chaining, and how does it resolve interrupt priority?

⚠️ MCQ Trap

NMI is reserved for critical events (e.g., hardware failure) and cannot be disabled by software — a frequently tested distinguishing fact.