ICT - Notes - intro to computer programing, Lecture notes of Information and Communications Technology (ICT)

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Introduction to Computer

Programming

Outline of Topics

• (^) Hardware/Software interface
  • (^) Layers of the Machine
  • (^) Kinds of Software
• (^) Computer Languages
• (^) Syntax, Semantics, Grammars
• (^) What happens to your program?
  • (^) Compilation, Linking, Execution
  • (^) Program errors
• (^) Compilation vs. Interpretation etc.

A Layered View of the Computer

Machine with all its hardware System Software

Compilers, Interpreters,Preprocessors,

etc.

Operating System, Device Drivers

Application Programs

Word-Processors, Spreadsheets,

Database Software, IDEs,

etc…

Operating System (OS)

 Provides several essential services:

• (^) Loading & running application programs
• (^) Allocating memory & processor time
• (^) Providing input & output facilities
• (^) Managing files of information

Computer Languages

• (^) Machine Language
  • (^) Uses binary code
  • (^) Machine-dependent
  • (^) Not portable
• (^) Assembly Language
  • (^) Uses mnemonics
  • (^) Machine-dependent
  • (^) Not usually portable
• (^) High-Level Language (HLL)
  • (^) Uses English-like language
  • (^) Machine independent
  • (^) Portable (but must be compiled for different platforms)
  • (^) Examples: Pascal, C, C++, Java, Fortran,...

Machine Language

• (^) The representation of a computer program which is actually read and understood by the computer. - (^) A program in machine code consists of a sequence of machine instructions.
• (^) Instructions:
  • (^) Machine instructions are in binary code
  • (^) Instructions specify operations and memory cells involved in the operation Example: Operation^ Address 0010 0000 0000 0100 0100 0000 0000 0101 0011 0000 0000 0110

High-level language

• (^) A programming language which use statements consisting of English-like keywords such as "FOR", "PRINT" or “IF“, ... etc.
• (^) Each statement corresponds to several machine language instructions (one-to-many correspondence).
• (^) Much easier to program than in assembly language.
• (^) Data are referenced using descriptive names
• (^) Operations can be described using familiar symbols
• Example: Cost := Price + Tax

Syntax & Semantics

• (^) Syntax:
  • (^) The structure of strings in some language. A language's syntax is described by a grammar.
  • (^) Examples:
    • (^) Binary number <binary_number> = | <binary_number> = 0 | 1
    • (^) Identifier = { | } = a | b |... | z <digit = 0 | 1 |... | 9
• (^) Semantics:
  • (^) The meaning of the language

Compilers & Programs

• Compiler

• (^) A program that converts another program from

some source language (or high-level

programming language / HLL) to machine

language (object code).

• (^) Some compilers output assembly language

which is then converted to machine language by

a separate assembler.

Compilation into Assembly L

Compiler Assembler Source Program Assembly Language Assembly Language Machine Language

Compilers & Programs

• Object program

• (^) Output from the compiler
• (^) Equivalent machine language translation of the

source program

• (^) Files usually have extension ‘.obj’

• Executable program

• (^) Output from linker/loader
• (^) Machine language program linked with necessary

libraries & other files

• (^) Files usually have extension ‘.exe’

What is a Linker?

• (^) A program that pulls other programs together so

that they can run.

• (^) Most programs are very large and consist of

several modules.

• (^) Even small programs use existing code provided

by the programming environment called libraries.

• (^) The linker pulls everything together, makes sure

that references to other parts of the program

(code) are resolved.

Program Execution

• Steps taken by the CPU to run a program

(instructions are in machine language):

1. Fetch an instruction

2. Decode (interpret) the instruction

3. Retrieve data, if needed

4. Execute (perform) actual processing

5. Store the results, if needed

Program Errors

• (^) Syntax Errors:
  • (^) Errors in grammar of the language
• (^) Runtime error:
  • (^) When there are no syntax errors, but the program can’t complete execution - (^) Divide by zero - (^) Invalid input data
• (^) Logical errors:
  • (^) The program completes execution, but delivers incorrect results
  • (^) Incorrect usage of parentheses