​Description of terms: assembler, compiler, interpreter, source program, object program

1. ​Description of terms
   1. assembler
   2. compiler
   3. interpreter
   4. source program
   5. object program

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​LANGUAGE TRANSLATORS

​A computer uses & stores information in binary form, and therefore, it cannot understand programs written in either high-level or low-level languages.  This means that, any program code written in Assembly language or high-level language must be translated into Machine language, before the computer can recognize & run these programs.  
A Translator is special system software used to convert the Source codes (program statements written in any of the computer programming languages) to their Object codes (computer language equivalents).

The Translators reside in the main memory of the computer, and use the program code of the high-level or Assembly language as input data, changes the codes, and gives the output program in machine-readable code.
In addition, translators check for & identify some types of errors (e.g., Syntax/grammatical errors) that may be present in the program being translated.  They will produce error messages if there is a mistake in the code.

Each language needs its own translator.  Generally, there are 3 types of language translators:

1. Assembler.
2. Interpreter.
3. Compiler.

Note. Interpreters & Compilers translate source programs written in high-level languages to their machine language equivalents.

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Assembler

​An assembler translates programs written in Assembly language into machine language that the computer can understand and execute.

​Functions of an Assembler.

1. It checks whether the instructions written are valid, and identifies any errors in the program.
   The Assembler will display these errors as well as the complete source and object programs.  If the program has no errors, the job control will let it run immediately, or save the object program so that it may run it later without translating it again.
2. It assigns memory locations to the names the programmer uses.E.g., the Assembler keeps a table of these names so that if an instruction refers to it, the Assembler can easily tell the location to which it was assigned.
3. It generates the machine code equivalent of the Assembly instructions.
   Usually, the Assembler generates a machine code only when no errors are detected.  Some of the errors include; 
   - Typing mistakes. 
   - Using the wrong format for an instruction.
   - Specifying a memory location outside the range 0 – 2047.

Note.  The Assembler cannot detect Logic errors.  The programmer knows of these errors only when the program is run & the results produced are incorrect (not what the programmer expected). The programmer must therefore, go through the program & try to discover why an incorrect result was being produced.

Compiler

​A compiler translates the entire/whole source program into object code at once, and then executes it in machine language code.  These machine code instructions can then be run on the computer to perform the particular task as specified in the high-level program.

The process of translating a program written in a high-level source language into machine language using a compiler is called Compilation.

For a given machine, each language requires its own Compiler.  E.g., for a computer to be able translate a program written in FORTRAN into machine language; the program must pass through the FORTRAN compiler (which must ‘know’ FORTRAN as well as the Machine language of the computer).

The object code file can be made into a fully executable program by carrying out a Linking process, which joins the object code to all the other files that are needed for the execution of the program.  After the linking process, an executable file with an .EXE extension is generated.  This file is stored on a storage media.

Points to note.

• The job of a Compiler is much more difficult than that of an Assembler in that, a single statement in a high-level language is equivalent to many machine instructions.
• The format of an Assembly instruction is fairly fixed, while high-level languages give a lot of freedom in the way the programmer writes statements.

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​Functions of a compiler.

A Compiler performs the following tasks during the compilation process:

1. It identifies the proper order of processing, so as to execute the process as fast as possible & minimize the storage space required in memory.
2. It allocates space in memory for the storage locations defined in the program to be executed.
3. It reads each line of the source program & converts it into machine language.
4. It checks for Syntax errors in a program (i.e., statements which do not conform to the grammatical rules of the language).  If there are no syntax errors, it generates machine code equivalent to the given program.
5. It combines the program (machine) code generated with the appropriate subroutines from the library.
6. It produces a listing of the program, indicating errors, if any.

​Differences between Compilers and Interpreters

Interpreter

​Interpreter

1. Translates & executes each statement of the source code one at a time.
   The source code instruction is translated & immediately obeyed by the computer hardware before the next instruction can be translated.
   (Translation & execution go together).
2. Translates the program each time it is needed for execution; hence, it is slower than compiling.
3. Interpreted object codes take less memory compared to compiled programs.
4. 4. For an Interpreter, the syntax (grammatical) errors are reported & corrected before the execution can continue.
5. An Interpreter can relate error messages to the source program, which is always available to the Interpreter.  This makes debugging of a program easier when using an Interpreter than a Compiler.

​Compiler

1. Translates all the source code statements at once as a unit into their corresponding object codes, before the computer can execute them.
   A Compiler translates the entire source program first to machine code, and then the code is executed by the CPU.
   (Translation & execution are separate phases)
2. Compiled programs (object codes) can be saved on a storage media and run when required; hence executes faster than interpreted programs.
3. Compiled programs require more memory as their object files are larger.
4. For a Compiler, the syntax errors are reported & corrected after the source code has been translated to its object code equivalent.
5. Once the source program has been translated, it is no longer available to the Compiler, so the error messages are usually less meaningful.  

​An interpreter translates a source program word by word or line by line.  This allows the CPU to execute one line at a time.

The Interpreter takes one line of the source program, translates it into a machine instruction, and then it is immediately executed by the CPU.  It then takes the next instruction, translates it into a machine instruction, and then the CPU executes it, and so on.

The translated line is not stored in the computer memory.  Therefore, every time the program is needed for execution, it has to be translated.

Source Program

​Source program (source code)

The term Source program refers to program statements that the programmer enters in the program editor window, and which have not yet been translated into machine-readable form.

Source code is the code understood by the programmer, and is usually written in high-level language or Assembly language.

Object Code

Object code (object program).

The term Object code refers to the program code that is in machine-readable (binary) form.

This is the code/language the computer can understand, and is produced by a Compiler or Assembler after translating the Source program into a form that can be readily loaded into the computer.

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Specific Objectives

Definition of an operating system

Functions of an operating system

• Job scheduling
• Resource Control
• Input/Output handling
• Memory Management
• Error handling
• Interrupt handling

Types of Operating Systems

• Number of users
• i) single user
• ii) multi user
• Number of tasks i) single tasking ii) multi tasking
• Interface
• i) Command line
• ii) menu driven interface
• iii) Graphical User Interface (GUI)

Organization of Information using an Operating System

• Files
• Directories/folders
• Storage media

File management using an Operating system

Description of files

Types of files

system files

application files

Functions of files

• i) storage of data
• ii) organization of information

Creating files

Manipulating files

• i) viewing files and directories
• ii) organization of information
• iii) creating files/directories
• iv) opening
• v) editing
• vi) renaming
• vii) fmding/searching
• viii) sorting
• ix) copying
• x) moving
• xi) deleting

Disk Management using an Operating system

• Formatting
• Partitioning
• Defragmentation
• Disk Diagnostics/Disk Compression
• Back up

Devices under Operating System Control

• Processor
• Memory (Ram)
• Storage devices
• Input/Output devices and ports
• Communication devices and ports

Installation and Configuration of an Operating system

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