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Any program written in a high-level language is known as source code. However, computers cannot understand source code. Before it can be run, source code must first be translated into a form which a computer understands - this form is called object code.

A translator is a program that converts source code into object code. Generally, there are three types of translator:

• compilers
• interpreters
• assemblers

Compilers

A compiler takes the source code as a whole and translates it into object code all in one go. Once converted, the object code can be run at any time. This process is called compilation.

All of the object files used in a program must be combined before the program can be run. This is done using a linker tool, which takes one or more objects and groups them into a single executable or a library.

Compilers have several advantages:

• Compiled programs run quickly since they have already been translated.
• A compiled program can be supplied as an executable file. An executable file is a file that is ready to run. Since an executable file cannot be easily modified, programmers prefer to supply executables rather than source code.
• Compilers optimise code. Optimised code can run quicker and take up less memory space.

Compilers have several disadvantages:

• Because the source code is translated as a whole, there must be enough memory space to hold the source code, the compiler and the generated object code. There also needs to be temporary working space for the compiler to perform the translation. Modern systems either have enough memory or use virtual memory to hold all the data.
• Compilers do not usually spot errors - the program has to be compiled and run before errors are encountered. This makes it harder to see where the errors lie.
• The source code must be recompiled every time the programmer changes the program.
• Source code compiled on one platform will not run on another - the object code is specific to the processor's architecture.

Interpreters

An interpreter translates source code into object code one instruction at a time. It is similar to a human translator translating what a person says into another language, sentence by sentence. The resulting object code is then executed immediately. The process is called interpretation.

Interpreters have several advantages:

• Instructions are executed as soon as they are translated.
• Since instructions are executed once translated, they are not stored for later use. As a result, interpreters require less available memory.
• Errors can be spotted quickly. Once an error is found, the program stops running and the user is notified at which part of the program the interpretation has failed. This makes interpreters extremely useful when developing programs.

Interpreters also have several disadvantages:

• Interpreted programs run more slowly. The processor has to wait for each instruction to be translated before it can be executed.
• Additionally, the program has to be translated every time it is run.
• Interpreters do not produce an executable file that can be distributed. As a result, the source code program has to be supplied and this could be modified without permission.
• Interpreters do not optimise code - the translated code is executed as it is.

Assemblers

Assemblers are a third type of translator. The purpose of an assembler is to translate assembly language into object code. Whereas compilers and interpreters generate many machine code instructions for each high-level instruction, assemblers create one machine code instruction for each assembly instruction.

Loader

A loader is a piece of software that is responsible for loading executable files into memory to enable the central processing unit (CPU) to run them. It calculates the size of a program and dedicates enough memory space for it to run efficiently.