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 PROCESS INVOLVED IN PROGRAM EXECUTION:

The process of program execution involves a series of steps that transform human-readable source code into instructions that a computer's central processing unit (CPU) can understand and carry out. These key stages are compilation, interpretation, linking, and loading.

Let's break down each step:

1. Compilation

What it is: Compilation is the process of translating an entire program, written in a high-level programming language (like C, C++, Java), into a lower-level language, typically machine code (binary instructions directly understood by the CPU) or intermediate code (like Java bytecode). This translation happens before the program is run.

How it works: A specialized program called a compiler performs this translation. The compiler reads the entire source code and goes through several analytical and synthesis phases:

• Lexical Analysis: Breaks the source code into basic units called "tokens" (e.g., keywords, identifiers, operators).

• Syntax Analysis (Parsing): Checks if the sequence of tokens conforms to the grammar rules of the programming language, creating a parse tree.

• Semantic Analysis: Checks for logical errors and type compatibility (e.g., ensuring you're not trying to add text to a number).

• Intermediate Code Generation: Creates an intermediate representation of the code, which is often more abstract than machine code but easier to optimize.

• Code Optimization: Improves the intermediate code for better performance (e.g., faster execution, less memory usage).

• Code Generation: Translates the optimized intermediate code into the target machine code or assembly code.

Output: The primary output of compilation is typically object code (e.g., files with .o or .obj extensions). This object code contains machine instructions for a specific part of the program, but it's not yet a complete, executable program because it might have unresolved references to other parts of the program or external libraries.

Characteristics:

• Pre-execution Translation: The entire program is translated before any part of it is run.

• Faster Execution: Compiled programs generally run much faster because the translation overhead occurs only once.

• Early Error Detection: Most syntax errors and some semantic errors are caught during compilation, preventing runtime crashes due to these issues.

• Platform Specific: The generated machine code is usually specific to a particular CPU architecture and operating system.

2. Interpretation

Interpretation is an alternative method of executing a program where the source code is translated and executed line by line or statement by statement at runtime, without generating a separate, complete machine code file beforehand.

How it works: An interpreter program directly reads and executes instructions from the source code. It reads a line, translates it into machine code (or an internal representation), and immediately executes it. This process repeats for each line of the program as it is encountered.

Output: There is no standalone executable file created. The output is the direct result of the program's execution.

Characteristics:

• Runtime Translation: Translation and execution occur concurrently.

• Slower Execution: Generally slower than compiled programs due to the repeated translation process during execution.

• Dynamic and Flexible: Easier for rapid prototyping, testing, and debugging, as changes can be made and run immediately.

• Greater Portability: The same source code can often run on different platforms as long as an interpreter for that language is available on each platform.

• Examples: Python, JavaScript, Ruby, PHP.

Note: Some languages (like Java) use a hybrid approach where source code is compiled into an intermediate bytecode (platform-independent), which is then interpreted by a Virtual Machine (JVM). Modern interpreters often use Just-In-Time (JIT) compilation, where frequently executed parts of the code are compiled into native machine code at runtime for performance.

3. Linking

Linking is the process of combining various compiled object code files and necessary libraries into a single, complete, executable program.

How it works: A program called a linker performs this crucial step. When you compile a program, it rarely exists in a single source file. Moreover, programs often use pre-written functions (e.g., for input/output, mathematical operations) from libraries (collections of compiled code).

The linker's main tasks are:

• Symbol Resolution: Resolving all symbolic references (like calls to functions or references to global variables) that are defined in one object file but used in another, or in a library. It finds the actual memory addresses for these symbols.

• Relocation: Adjusting memory addresses within the object files so that they refer to the correct locations when combined into a single executable file in memory.

Types of Linking:

• Static Linking: The actual machine code for all necessary library functions is copied directly into the final executable file. This makes the executable self-contained but generally larger.

• Dynamic Linking: Instead of copying library code, the linker simply records the names of the libraries and the functions required. The actual linking to the shared libraries (e.g., .dll on Windows, .so on Linux) happens at runtime. This results in smaller executable files and allows multiple programs to share a single copy of a library in memory, saving resources.

Output:

A complete executable file (e.g., .exe on Windows, or just the program name on Linux/Unix), which is ready to be loaded into memory and run.

4. Loading

Loading is the final step in preparing a program for execution. It's the process of transferring the executable file from secondary storage (like a hard disk drive or SSD) into the computer's main memory (RAM).

How it works:

A component of the operating system called the loader performs this function. When a user initiates a program (e.g., by double-clicking an icon or typing a command), the OS loader:

• Allocates Memory: Finds and reserves a contiguous block of available memory in RAM for the program and its data.

• Copies Code and Data: Reads the executable file from disk and copies its machine code instructions and initial data into the allocated memory.

• Sets Up Program Context: Initializes CPU registers (like the Program Counter to point to the program's starting instruction, and the Stack Pointer for runtime memory management).

• Handles Dynamic Linking (if applicable): If the program is dynamically linked, the loader (or a dynamic linker/loader component) identifies and loads the required shared libraries into memory if they are not already present, resolving their addresses.

• Transfers Control: Once everything is in place, the loader transfers control of the CPU to the program's entry point (often the main function in C/C++/Java programs), and the program begins to execute its instructions.

Output: The program is now running in the computer's RAM, and the CPU is actively executing its instructions.