---
title: "Software reliability; waterfall vs prototyping; embedded software"
subject: "Instrumentation II"
chapter: 9
key: "semester_5/instrumentation_ii:sw-reliability-models-embedded"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/sw-reliability-models-embedded
site: BCT Engineering
---

# Software reliability; waterfall vs prototyping; embedded software

Instrumentation II · Chapter 9: Software For Instrumentation And Control Applications

- Asked in: 2075 Asoj, 2+3+3 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/sw-reliability-models-embedded
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-9

## Question

What is software reliability? Compare waterfall and prototyping model. Describe Embedded and Real Time Software.

## Short answer

Software reliability is the probability of failure-free operation for a specified time and environment. Waterfall follows sequential phases with fixed requirements, while prototyping refines through customer feedback and early versions. Embedded software is dedicated firmware in a device with limited resources; real-time software must meet strict timing deadlines, often using interrupts and a RTOS.

## Answer

#### Software reliability
**Software reliability** is the probability that software performs its required function without failure for a specified time in a specified environment. It is improved by testing and bug removal, and is measured by failure rate, MTBF and defect density. Unlike hardware, software does not wear out; failures are due to design faults.

#### Waterfall vs prototyping
| Point | Waterfall | Prototyping |
|---|---|---|
| Process | Sequential phases | Build, evaluate, refine prototype |
| Requirements | Fixed at start | Evolve with feedback |
| Customer involvement | Mostly at start and end | Throughout |
| Working version | Late | Early |
| Risk | High if requirements are wrong | Lower |
| Documentation | Good | Often poor |
| Cost of change | High | Lower in early stage |
| Best for | Clear, stable requirements | Unclear requirements, user interfaces |

#### Embedded and real-time software
**Embedded software** is a program stored in the memory of a microcontroller/processor built into a device for a dedicated function, interacting directly with the hardware, and with limited resources. **Real-time software** must respond to events and produce results within strict time deadlines (hard or soft), using interrupts, timers and often a real-time operating system. Most instruments are both: embedded real-time software.

## Check yourself

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

1. Software reliability is?
   A. Speed of execution
   B. Code length
   C. Probability of failure-free function for a specified time in a specified environment
   D. Number of users
   Answer: C. Standard definition. (Key point: Reliability)

2. Unlike hardware, software?
   A. Wears with age
   B. Fails from friction
   C. Needs lubrication
   D. Does not wear out; failures are design faults
   Answer: D. Failures come from faults. (Key point: Wear)

3. Which has requirements evolving with feedback?
   A. Prototyping
   B. Waterfall
   C. Both fixed
   D. Neither
   Answer: A. Waterfall fixes them at the start. (Key point: Waterfall vs prototyping)

4. Real-time software must?
   A. Be large
   B. Respond within strict deadlines
   C. Use no interrupts
   D. Have no hardware link
   Answer: B. Timing is part of correctness. (Key point: Real-time)

## Related in Instrumentation II

- [Increasing reliability of embedded real-time software](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aembedded-rt-reliability) — Instrumentation II, chapter 9
- [Embedded real-time software; model for project](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aembedded-rt-software-model) — Instrumentation II, chapter 9
- [Prototyping model of software](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aprototype-model) — Instrumentation II, chapter 9, asked 3×
- [Compare software models; propose a fourth model](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Asw-models-compare-fourth) — Instrumentation II, chapter 9
- [Spiral model of software development](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aspiral-model) — Instrumentation II, chapter 9, asked 3×

## Related in other subjects

- [Software runaways; waterfall and prototyping inside spiral](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Asoftware-runaways-spiral) — Software Engineering, chapter 1
- [Spiral model of embedded software life cycle](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aspiral-model-esdlc) — Embedded System, chapter 3
- [Real-time versus ordinary software and OS; data acquisition](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Areal-time-vs-non-real-time) — Software Engineering, chapter 4
- [Designing software for real-time environments](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Areal-time-design-considerations) — Software Engineering, chapter 4