Computer Science: Chapter 6 & 7.

Computer Science: Chapter 6 & 7.

1. Principles of programming

   • A high-level language is a programming language that allows code to be written. It is similar to a natural human language, such as English. Some programmers prefer to use high-level programming languages, as they are easier to understand, learn and program. Their commands are similar to natural languages like English and identifiers can be long and meaningful.
   • Examples of common high-level programming languages include:
     • Basic
     • Java
     • Pascal
   • Machine code is the opposite of a high-level language in that it does not resemble any natural language and is made up entirely of bit patterns (instructions or data) that can be executed directly by the CPU. High-level languages must be converted into machine code before they can be executed by the CPU.
   • Examples of machine code instructions are opcodes and operands.
   • Programming in a low-level language, such as assembly code requires knowledge of the internal structure of the CPU and is therefore very specialized. The program statements are written for a particular type of CPU and make direct reference to specific internal registers.
   • Assembly code uses mnemonics and is converted to machine code for execution using an assembler. Source code produced in a low-level language is not portable, but it can be very efficient and the programs can be made to run faster than programs produced using a high-level language.
   • High-level languages are used when the execution speed is not the most critical factor, e.g. in common productivity applications, such as a word processor, or spreadsheet. Most modern applications such as commercial database packages, operating systems, e-commerce software and social media apps are developed using a high-level programming language. Although uncommon, some programmers may wish to program directly in machine code or use assembly code. This is primarily done when programming device drivers or embedded systems, where fast execution speeds are critical. Professional game developers may need to use console specific development software, which is likely to include low-level features for optimum performance.

2. Software Engineering

   • Software development environments, also known as Integrated Development Environments (IDE), provide programmers with various tools that are needed to create computer programs.
   • Here are some of the tools and facilities offered by a typical software development environment:
     • Editor
       • Allows a programmer to enter, format and edit source code
     • Compiler
       • Converts source code into executable machine code. Once compiled, a program can be run at any time
     • Interpreter
       • Converts each line of source code into machine code, and executes it as each line of code is run. The conversion process is performed each time the program needs to be run
     • Linker
       • A program which allows previously compiled code, from software libraries, to be linked together
     • Loader
       • A program which loads previously compiled code into memory
     • Debugger
       • A program which helps locate, identify and rectify errors in a program
     • Trace
       • A facility which displays the order in which the lines of a program are executed, and possibly the values of variables as the program is being run
     • Break Point
       • A facility which interrupts a program on a specific line of code, allowing the programmer to compare the values of variables against expected values. The program code can then usually be executed one line at a time. This is called single-stepping
     • Variable Watch
       • A facility that displays the current value of any variable. The value can be 'watched' as the program code is single-stepped to see the effects of the code on the variable. Alternatively a variable watch may be set, which will interrupt the program flow if the watched variable reaches a specified value
     • Memory Inspector
       • A facility which will display the contents of a section of memory
     • Error Diagnostics
       • Used when a program fails to compile or to run. Error messages are displayed to help the programmer diagnose what has gone wrong
   • Libraries
     • A library is a collection of commonly used private functions and subprograms. Examples of private functions include standard mathematical operations such as square root, random number generators. Examples of subprograms include standard input/output routines, such as saving data to disk. These functions and subprograms can be called from within your program at any stage, but only when the appropriate library has been linked.
     • The advantages of using standard libraries are that:
       • related private functions and subprograms are stored in the same location
       • time is saved as the programmer can simply use the private functions and programs stored in a library
       • subroutines contained in a library have already been tested, so they should work reliably and not need further testing
       • programs will contain less code and will therefore be easier to maintain