GNVQ Advance IT- Unit 6 


Unit 6 - Element 6.2 - Production of Software

All software, whether systems or application software is a series of instructions to the computer which it can understand and obey.   Initially, these instructions are written in a programming language by a computer programmer.  Over the years these languages and instructions have evolved as technology has advanced and as the use of computers has become more widespread.   Essentially, there are three levels of language: High level, low level and machine code.

Machine Code
Internally, any computer only understands instructions in machine code and these are normally written using binary. An example of machine code is shown below.

11011101  1011011
01001100  1011100
11011100  1011011

 In the very early days of computing these instructions would be given to the computer by setting a series of switches to either on or off which would represent the bit settings.  Later, the instructions would have been written in decimal notation, punched onto paper tape or cards and then translated into binary as the card or tape was read into the computer.  Even so, this is a very complex and complicated way of writing programs, and means a great deal of time must be spent in training programmers.   It then takes a programmer a long time to produce a working program.

 The instruction set depends on the internal architecture of the machine, in other words what processor the machine uses.  So programs written in machine code are not 'portable' to different brands of computer.


Low Level Languages or Assembly Languages
These languages evolved from machine code, in that they are also machine dependant and each assembly language instruction represents one machine code instruction, but the instructions are written in mnemonics ( 3 digit abbreviations).  An example is shown below:

LDA 5000
ADA 6000
STA 5000

These three lines of code add two numbers together and store the result in the memory location of the first one.  The code does not take into account the size of the numbers as there is nothing to check whether a carry has taken place.  So it is still a complex process to write a program in assembly language and it is still not a portable language.  However, it is easier to remember that LDA stands for ‘load accumulator’ than to remember a string of binary digits.


High Level Languages
High level languages use instructions that are much more like English.  This enables a programmer to learn the language more quickly and more easily and to produce a working program in a shorter space of time.

Different high  level languages were written to be used in differing situations.

COBOL was used for applications that were intended to process large amounts of data.

FORTRAN was used in scientific programs, where complex calculations were necessary and where it was required to store data to a high level of accuracy.

BASIC was used as a teaching language, although it has some very powerful features.

The C and C++ languages can be used to write systems software.

PASCALwas written to encourage structured programming.  An example of PASCAL code is given below:

Read(TaxablePay);
IF TaxablePay < 4000 THEN
tax:=0
ELSE
tax:= TaxRate*TaxablePay;
{end if}
Write(tax:6:2);

High level languages are not deigned for any particular computer but are what are called ‘portable’.  Once you have learned to program in a high level language you will be able to transfer your skills to different types of computer.


Language Translation
Computers can only understand instructions that are in machine code.  Therefore all high and low level languages need to be translated into machine code before the computer can execute the instructions.

For low level or assembly languages these translators are called assemblers and they translate each instruction  into one machine code instruction.

High level languages are translated by compilers or interpreters and will produce many machine code instructions for each instruction in the original language.

The original code written by the programmer is known as the source code and the translated  code that has been produced by the compiler is called the object code.  When all the object code modules have been linked together and any library routines included, we then have an executable program.


 
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