How Computers and Phones Actually Boot — From Power Button to Operating System
Most people press the power button on a computer or phone and simply wait for the screen to light up.
Software engineers should know what actually happens in between.
This article explains — in clear, human terms — how computers and mobile devices boot, from the very first electrical signal to a fully running operating system.
No magic. No hand-waving. Just solid fundamentals.
The Moment You Press the Power Button
When you press the power button on a computer or mobile device, you are not “starting the operating system.”
You are starting an electrical process.
An electrical signal travels from the power source (battery or wall outlet) to the motherboard, the main electronic board of the device.
This signal is interpreted as high and low electrical pulses, which computers understand as bits (0 and 1).
The First Code That Ever Runs
Those electrical signals reach a special chip on the motherboard whose job is to start the system.
This code is firmware, stored permanently in hardware.
- Computers: UEFI or legacy BIOS
- Android: Primary Boot Loader (PBL)
- iPhone: Secure ROM → iBoot
This firmware is trusted, minimal, and runs before anything else.
POST — Power-On Self-Test
Before loading any operating system, the firmware runs POST, verifying:
- Display
- Keyboard
- Ports
- Core hardware components
Errors here result in beeps or startup failures.
Finding the Operating System
If POST passes, the firmware locates boot instructions in permanent storage:
- Disk drives (PCs)
- Soldered flash chips (phones, tablets)
Control is handed to the boot loader, which loads the operating system.
The CPU Takes Control
The OS code is sent to the CPU, which executes low-level assembly instructions.
At this stage:
- Graphics
- Audio
- Input
- Core services
are initialized, using RAM for fast, temporary storage.
The Kernel — The Core of the System
The kernel is loaded and takes control.
It manages:
- Memory
- Processes
- CPU scheduling
- Devices
- Files
Applications never access hardware directly — they go through the kernel.
Users, Security, and Access Control
Once the kernel is running:
- User authentication starts
- Permissions are enforced
- Encrypted data is unlocked after verification
This ensures privacy and system integrity.
Drivers — Talking to Hardware
Hardware devices require drivers, which allow the OS to communicate with:
- Displays
- Keyboards
- Storage
- GPUs
- Sensors
Without drivers, hardware cannot function.
Core Computing Concepts
🧠 Basics
- Bit → smallest unit of data
- Pixel → smallest display unit
- Hardware → physical components
- Software → logical programs
⏩ Boot Components
- BIOS / UEFI
- POST
- Boot Loader
💾 System Components
- RAM
- Storage
- CPU
- GPU
- SoC
🧰 Low-Level Software
- Kernel
- Drivers
- Assembly language
Why This Matters
Understanding boot processes gives engineers insight into:
- Performance
- Security
- Stability
- System design
This knowledge applies to computers, phones, IoT devices, and servers.
What’s Next
Future parts will explore:
- Processes & threads
- Memory models
- Kernel vs user space
- Scheduling and performance
✍️ Written by Cristian Sifuentes — software engineering fundamentals, systems thinking, and architecture.
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