CCEA A-Level Software Systems Development

Software falls into two broad groups. Systems software runs the computer and provides the services other programs depend on. Application software does a job for the user, and application development software is the set of tools a programmer uses to build it.

Systems software

The operating system is the main piece of systems software. It manages the processor, memory and storage, runs programs, controls input and output devices through device drivers, and provides the file system. Alongside it sit utility programs (disk tools, backup, anti-virus, compression) and the translators that turn a programmer's code into something the processor can run.

Managing files

The operating system organises stored data as files in a hierarchy of folders. It records each file's name, type (often shown by its extension), size, dates and permissions, and it decides who may read, change or run the file. A program that opens a file asks the operating system to do it, which is why a program can fail with a "file not found" or "access denied" error that has nothing to do with its own logic.

Four kinds of file matter in software development.

A source file is translated by a compiler into a binary file. In the .NET platform the compiler produces an assembly, which contains code in an intermediate language (not machine code) and is either an executable (.exe) or a library (.dll). When the program runs, the runtime translates the intermediate code to machine code for the processor it is on. Java works in a similar way, with bytecode run by the Java Virtual Machine. The benefit is that the same compiled program can run on different hardware that has the platform installed.

Translators

A compiler translates the whole program into another form before it runs, and reports syntax errors at that point. An interpreter translates and runs one statement at a time. An assembler translates assembly language into machine code. Compiled programs generally run faster because the translation has already been done, while interpreted programs are easier to test a line at a time.

XML

XML (Extensible Markup Language) stores data as nested elements with a name, written between opening and closing tags.

<student>
  <name>Aoife Brennan</name>
  <year>13</year>
  <subjects>
    <subject>Software Systems Development</subject>
    <subject>Mathematics</subject>
  </subjects>
</student>

Every opening tag needs a matching closing tag, and elements must be nested properly. XML is plain text, so it can be read in any editor and moved between different systems, and the tags make it self-describing. It is used for configuration settings, for exchanging data between programs and for saving structured data. Because every value is wrapped in tags, an XML file is larger than the same data in a binary file, and it is slower to read.

Application development software

Application development software is what a programmer uses to write, build and test an application.

• An integrated development environment (IDE) such as Visual Studio brings an editor, a compiler, a debugger and a form designer into one program. The editor colours the syntax and suggests completions, and the debugger lets the programmer stop a running program at a chosen line, step through it and watch the variables change.
• A class library is a large collection of tested classes, such as those for files, strings and collections, which the programmer reuses instead of writing them again.
• Version control records each change to the source files, so that earlier versions can be recovered and several developers can work on the same code.

Object oriented platforms

An object oriented platform is an environment that supports building programs out of classes and objects. It supplies the language, the class library, the compiler and the runtime. .NET (with C#) and Java are the two common examples. The platform takes care of tasks such as memory management, so the programmer does not have to release memory by hand: the runtime finds objects that are no longer referred to and reclaims their memory, which is called garbage collection.

Open source software

Open source software is distributed with its source code, under a licence that lets people use, study, change and share it. Linux, the Java development kit (OpenJDK) and the Python interpreter are open source, and .NET itself is open source.