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What Really Happens When a Program Runs?

· 3 min read

🎯 Why This Topic Matters​

Every computer science student writes code, but very few truly understand what happens after pressing “Run”.

Whether you write C, C++, Python, or Java — the journey from source code to running program involves the compiler, operating system, memory, and CPU working together.

Understanding this gives you: ....

  • Strong OS fundamentals
  • Better debugging skills
  • Clear answers in interviews
  • A real mental model of how computers work

1️⃣ Writing the Source Code​

This is the part everyone knows.

#include <stdio.h>

int main() {
    printf("Hello, Binary Dose!");
    return 0;
}

At this stage:

  • It is human-readable
  • The CPU cannot understand it
  • It is just a text file on disk

2️⃣ Compilation vs Interpretation​

What happens next depends on the language.

Compiled Languages (C, C++)​

  • Source code → machine code before execution
  • Errors caught early
  • Faster execution

Interpreted Languages (Python, JavaScript)​

  • Code executed line by line (conceptually)
  • More flexible
  • Slightly slower

⚠️ Note: Modern languages often use hybrid models (bytecode + virtual machine).

3️⃣ Creating an Executable File​

For compiled languages:

source code → object file → executable

The executable contains:

  • Machine instructions
  • Metadata (entry point, sections)
  • Information required by the OS loader

At this point, still nothing is running.

4️⃣ OS Loads the Program into Memory​

When you run the program:

  • The OS creates a process
  • A unique Process ID (PID) is assigned
  • Virtual memory is allocated
  • Required libraries are linked

5️⃣ Program Memory Layout​

Each process gets its own virtual address space, divided into regions:

🧩 Typical Memory Segments

SegmentPurpose
TextProgram instructions
DataGlobal & static variables
HeapDynamic memory (malloc, new)
StackFunction calls & local variables

👉 This isolation is why one program cannot directly access another program’s memory.

6️⃣ CPU Starts Executing Instructions​

Now the real execution begins.

The CPU follows the instruction cycle:

Fetch → Decode → Execute → Repeat
  • Program Counter (PC) points to the next instruction
  • Instructions are fetched from memory
  • Results are stored in registers or memory
  • This continues until the program ends.

7️⃣ Program Termination​

When the program finishes:

  • return 0 (or equivalent) is executed
  • OS frees memory
  • File handles are closed
  • Process entry is removed

The system is now clean again.

⚠️ Common Misconceptions

  • ❌ “Program runs directly from disk” ✅ Programs run from RAM, not disk

  • ❌ “Page fault always means disk access” ✅ Page fault can occur even when data is already in memory

  • ❌ “Stack and heap are language features” ✅ They are runtime memory concepts

💡 Interview Perspective You may be asked:

  • What happens after main() starts?
  • Difference between process and program?
  • Why virtual memory is used?
  • How stack overflow occurs?

This topic connects OS, memory, and execution — a favorite interview area.

📌 Final Thought If you understand what happens when a program runs, you understand how computers actually work — not just how to write code.

This foundation will help you everywhere in computer science.