Applications Generation and Software Development

1 Describe the stages of compilation. 6

One mark for each of the following, to a maximum of 6.

• Lexical analysis groups characters into tokens
• Whitespace and comments are removed and the symbol table is created
• Syntax analysis parses tokens against the grammar of the language
• An abstract syntax tree is produced and syntax errors are reported
• Code generation converts the tree into machine or object code
• Optimisation improves the code so it runs faster or uses less memory

Model answer. Compilation begins with lexical analysis, in which the source is read character by character and grouped into tokens representing keywords, identifiers, operators and constants. Whitespace and comments are discarded because they have no meaning to the compiler, and identifiers and constants are recorded in the symbol table along with attributes such as type and scope. Syntax analysis then parses the stream of tokens against the formal grammar of the language, building an abstract syntax tree that represents the structure of the program, and reporting a syntax error for anything that cannot be fitted to the grammar. Semantic checks such as type compatibility and the use of undeclared identifiers are also carried out at this point. Code generation then walks the abstract syntax tree and produces the equivalent machine code or object code. Finally the optimisation stage improves that code, for example by removing instructions whose results are never used, moving calculations that do not change out of loops, and allocating registers more efficiently, so that the finished program runs faster or occupies less memory.

2 Explain the difference between static and dynamic linking. 4

One mark for each of the following, to a maximum of 4.

• Static linking copies the library code into the executable at compile time
• The executable is larger but self contained
• Dynamic linking stores only a reference and loads the library at run time
• The executable is smaller and library updates apply automatically, but a missing library breaks it

Model answer. Static linking copies the machine code of every library subroutine the program uses directly into the executable file at compile time. The result is a larger file, and identical library code is duplicated across every program that uses it, but the executable is entirely self contained: it will run wherever it is placed and cannot be broken by a later change to the library. Dynamic linking instead stores only a reference to the library, which is located and loaded into memory when the program runs. Executables are therefore much smaller, one copy of the library serves every program on the system, and a security fix or improvement to that library benefits all of them without any of them being rebuilt. The trade off is a run time dependency: if the library is missing, or has been replaced by an incompatible version, the program will fail to start or behave unpredictably.

3 A company is developing a control system for a medical device. The requirements are fixed by regulation and full documentation must be produced. Recommend a development methodology and justify your choice. 6

One mark for each of the following, to a maximum of 6.

• Recommends the waterfall methodology
• The requirements are fixed and fully known in advance
• Waterfall completes each stage fully before the next begins
• It produces thorough documentation at every stage, which regulation demands
• Its inflexibility is not a problem because the requirements cannot change
• Notes that a methodology such as agile would produce insufficient documentation for approval

Model answer. The company should use the waterfall methodology. Its central weakness, inflexibility in the face of changing requirements, does not apply here because the requirements are fixed by regulation and cannot change during development, so the risk of an expensive late revision is minimal. Waterfall's sequential structure, completing analysis, design, implementation, testing and evaluation fully in turn, produces a formal signed off deliverable at the end of each stage, and this is precisely what a regulator needs: a complete audit trail demonstrating that every requirement was specified, designed for, implemented and tested. In a medical device context this documentation is not overhead, it is a legal necessity, and its absence would prevent approval regardless of how good the software actually was. An agile approach would be a poor fit here even though it is often preferable elsewhere. Agile deliberately produces lighter documentation and expects requirements to evolve through customer feedback, neither of which is appropriate when the specification is externally imposed and immutable. Its strength, responding rapidly to change, has no value in this project. The one adjustment worth making is to include rigorous verification within each waterfall stage rather than leaving all testing to the end, since defects found late in a safety critical system are extremely expensive to correct.

4 Explain two advantages of using a library of pre-written subroutines. 4

One mark for each of the following, to a maximum of 4.

• Development time is reduced because the code does not have to be written
• The developer can concentrate on the parts specific to their application
• Library code is widely used and tested, so it is more reliable
• Specialist areas such as cryptography are extremely difficult to implement correctly

Model answer. The first advantage is development speed. A library provides subroutines for common tasks such as sorting, mathematical functions, file compression or network communication, so the developer does not have to write, debug and document that functionality themselves and can concentrate on the parts of the system that are specific to their own application. The second advantage is reliability. Library code has typically been used by very large numbers of developers over a long period, so defects have been found and corrected in a way that newly written code cannot match. This matters most in specialist areas such as cryptography, where a subtly incorrect implementation may appear to work perfectly while being completely insecure, and where writing it yourself is almost always the wrong decision.

5 Compare the waterfall and agile approaches to software development. 6

One mark for each of the following, to a maximum of 6.

• Waterfall is sequential with each stage completed before the next
• Agile is iterative, delivering working software in short repeated cycles
• Waterfall produces extensive documentation at every stage
• Agile involves the customer continuously and adapts to changing requirements
• Waterfall handles change poorly and shows the client nothing working until late
• Agile can be harder to cost and schedule and produces lighter documentation

Model answer. Waterfall is a sequential methodology in which analysis, design, implementation, testing and evaluation are each completed and signed off before the next begins, producing a full set of documentation at every stage. Agile is iterative: the project is divided into short cycles, each of which delivers a working increment of the software, and the customer reviews that increment and influences what is built next. This produces two very different profiles. Waterfall gives strong control and a complete audit trail, which suits projects where the requirements are fixed and where regulators or contracts demand documented evidence, but it handles change extremely badly, because altering a requirement after design is complete means revisiting earlier stages at considerable cost, and the client sees nothing that actually runs until very late in the project. Agile handles change as a normal part of the process rather than an exception, and because working software is delivered early and frequently, misunderstandings about requirements surface within weeks rather than months. Its costs are that it demands continuous customer availability, which many organisations cannot provide, that the final cost and delivery date are much harder to predict at the outset, and that its lighter documentation can make the system more difficult to maintain years later when the original developers have moved on.