[Day 2] 4. Executable File

!4. Executable File.png

Executable File Structure

An executable file contains several key sections, each serving a specific purpose:

1. Executable Header:

   • Contains information about the program, such as its name and size.
   • Specifies the type of program (e.g., CLI, GUI, library).
   • Provides the address of the entry point function.
   • Includes information about the remaining sections (start/end).
   • Contains a magic number:
     • Serves as an identifier for the file format.
     • Takes up 2 to 4 bytes of space.
   • Examples:
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       .out

       (Linux): Executable and Linkable Format (ELF)
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       .exe

       (Windows): Portable Executable (MZ)

2. TEXT Section:

   • Contains instructions in machine code format.
   • Also referred to as the code section.

3. Data Section:

   • Contains initialised static and global variables.
   • Example:
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     int num = 10;

4. BSS (Block Started by Symbol) Section:

   • Contains uninitialised static and global variables.
   • Example:
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     int num;

5. RO (Read Only) Data Section:

   • Contains read-only data, such as string constants.
   • Example:
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     char* ptr = "sunbeam";

6. Symbol Table:

   • Stores information about symbols.
   • Two types of symbols:
     1. Variables: Includes name, size, address, section, type, etc.
     2. Functions: Includes name, address, return type, and the number and type of arguments.

Process Execution

• The CPU cannot directly access the hard disk; it requires data to be loaded into RAM for faster access.
• The loader is responsible for transferring data from the hard disk to RAM while utilising information from the executable header and symbol table.

Memory Structure

• Stack:

  • Stores the Function Activation Record (FAR) and local variables.

• Heap:

  • Stores dynamically allocated space.

• Process Control Block (PCB):

  • Contains information required to control the execution of a process.
  • Stores Process ID (PID) and Parent Process ID (PPID).

Process from OS Perspective

From the operating system's perspective, a process consists of all its sections loaded into memory along with the PCB.

• The space where the OS is loaded is known as Kernel Space.
• The space where user applications run is referred to as User Space.

Commands for Process Management

• Whenever a command is executed, it is treated as a program that is first loaded into RAM, with a PCB created for it.

1. Top Command:

   • Displays active processes in a top-ordered list.

2. Echo Command:

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     echo $SHELL

     : Displays the current shell being used.

3. Process Status Commands:

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     ps

     : Displays the running processes of the current shell.
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     ps -e

     : Displays all running processes in RAM.
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     ps -e -o pid,ppid,cmd

     : Changes the format of the output to show specific details (PID, PPID, command).

4. TTY:

   • Stands for Terminal.