---
title: "Compare status check I/O, interrupt I/O and DMA"
subject: "Instrumentation II"
chapter: 1
key: "semester_5/instrumentation_ii:compare-status-interrupt-dma"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/compare-status-interrupt-dma
site: BCT Engineering
---

# Compare status check I/O, interrupt I/O and DMA

Instrumentation II · Chapter 1: Microprocessor Based Instrumentation System

- Asked in: 2078 Bhadra, 2+6 marks
- Video: [Computer Organization #66: Programmed IO | Interrupt Initiated I/O | Types of Interrupt |](https://www.youtube.com/watch?v=_GlKBqsQXrQ) — Vinay Mishra
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/compare-status-interrupt-dma
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-1

## Question

Define instrumentation system. Compare status check I/O, Interrupt driven I/O and DMA.

## Short answer

An instrumentation system measures, processes, displays and/or controls physical quantities. In status check I/O, the CPU polls the flag, causing high overhead and slow speed. Interrupt-driven I/O reduces CPU time by servicing only on request, while DMA uses a controller for block transfers with minimal CPU involvement and highest speed.

## Answer

An **instrumentation system** is a system of devices that measures, collects, processes, displays and/or controls physical quantities. In a microprocessor-based version, the three ways to transfer data between I/O and CPU are compared below.

| Point | Status check (polling) | Interrupt driven | DMA |
|---|---|---|---|
| Who controls transfer | CPU program | CPU, on request of device | DMA controller |
| CPU activity | CPU loops, reading the status flag | CPU does other work, services ISR when interrupted | CPU free, bus only held for short time |
| CPU overhead | Very high | Medium (context saving per byte) | Very low |
| Speed | Slow | Medium | Fastest (block rate near memory speed) |
| Hardware | Simplest: port and status bit | Interrupt lines, INTA, controller (8259) | DMA controller (8237), HOLD/HLDA |
| Response time | Depends on polling loop | Fast (latency of an instruction) | Very fast |
| Suitable for | Few, simple, slow devices | Occasional events, keyboards, alarms | Block transfers: disk, ADC bursts |
| Software | Simple | ISR needed | Needs set-up of address and count |

```text
 Polling:  loop: IN status; test ready? no -> loop; yes -> IN data
 Interrupt: main program ... <INTR> -> ISR -> return
 DMA:      DREQ -> HOLD -> HLDA -> block transferred -> HOLD off
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does polling have very high CPU overhead?
   A. The CPU loops reading the status flag
   B. Interrupts are slow
   C. DMA blocks it
   D. The CPU is idle
   Answer: A. Looping wastes CPU time. (Key point: Polling overhead)

2. Which hardware supports interrupt driven I/O?
   A. Interrupt lines, INTA and a controller such as 8259
   B. 8237 DMA controller
   C. Only a status port
   D. HOLD/HLDA
   Answer: A. 8259 manages interrupts; 8237 is for DMA. (Key point: Interrupt hardware)

3. Which tasks suit DMA?
   A. Occasional events like keyboards
   B. Block transfers such as disk and ADC bursts
   C. Few simple slow devices
   D. Single alarm
   Answer: B. Block rates near memory speed. (Key point: DMA suitable for)

4. Which is the correct speed order from slowest to fastest?
   A. DMA, interrupt, polling
   B. Interrupt, polling, DMA
   C. Polling, interrupt, DMA
   D. Polling, DMA, interrupt
   Answer: C. Polling is slow, interrupts medium, DMA fastest. (Key point: Speed order)

## Related in Instrumentation II

- [Interrupt driven data transfer](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ainterrupt-driven-transfer) — Instrumentation II, chapter 1, asked 2×
- [MBI block diagram, advantages and DMA](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-block-advantages-dma) — Instrumentation II, chapter 1
- [DMA concept; ROM/RAM interface at 4000H](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adma-memory-interface-4000h) — Instrumentation II, chapter 1
- [DMA active and idle cycles](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adma-cycles-disadvantages) — Instrumentation II, chapter 1
- [Interfacing 8-bit ADC using interrupt](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aadc-interrupt-interface) — Instrumentation II, chapter 4, asked 2×
- [Features of microprocessor based instrumentation](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-features) — Instrumentation II, chapter 1, asked 4×

## Related in other subjects

- [Programmed vs interrupt-driven vs DMA](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aio-techniques-compare) — Computer Organization and Architecture, chapter 7, asked 5×
- [DMA vs programmed and interrupt-driven I/O](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adma-overcomes) — Computer Organization and Architecture, chapter 7, asked 3×
- [Interrupt-driven I/O with vectored interrupt](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Avectored-interrupt-io-flow) — Embedded System, chapter 5, asked 3×
- [Instrumentation system with block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Ainstrumentation-system) — Basic Electronics Engineering, chapter 7, asked 3×