Linux Architecture¶

This note summarizes the basic architecture of a Linux system and the relationship between user applications, the kernel, and hardware.

Linux System Architecture¶

A Linux system can be divided into three main layers:

+----------------------+
|      User Space      |
| Applications / Shell |
+----------------------+
|        Kernel        |
| Drivers / Scheduler  |
+----------------------+
|       Hardware       |
| CPU / RAM / Devices  |
+----------------------+

Hardware Layer¶

The hardware layer includes the physical components of the system.

Examples:

CPU
RAM
UART
SPI
I2C
Network interfaces
Storage devices

For embedded systems, this is typically an SoC (System-on-Chip) containing the CPU and peripherals.

Example (MCU concept):

MCU
 ├── UART
 ├── SPI
 ├── GPIO
 ├── Timer
 └── Flash

Kernel¶

The Linux kernel is the core of the operating system.
It manages hardware resources and provides services to user applications.

Main responsibilities:

Process scheduling
Memory management
Device drivers
File systems
Networking stack
Inter-process communication (IPC)

For embedded developers, the kernel can be roughly compared to:

RTOS + Drivers + Networking + File System

However, Linux is much more complex and supports multi-user and multi-process environments.

User Space¶

User space contains applications and system utilities.

Examples:

bash
ssh
python
systemd
user applications

Example command:

python app.py

User programs cannot directly access hardware.

System Calls¶

User applications interact with the kernel through system calls.

Common system calls:

open()
read()
write()
close()
ioctl()

Example:

fd = open("/dev/ttyAMA10", O_RDWR);
write(fd, "hello", 5);

Execution flow:

User Application
      ↓
   System Call
      ↓
   Linux Kernel
      ↓
  Device Driver
      ↓
    Hardware

Everything is a File¶

A key design principle in Linux is:

Everything is represented as a file.

Examples:

/dev/ttyAMA10
/dev/spidev10.0
/dev/i2c-1
/dev/random

These files represent hardware devices and allow applications to communicate with them using standard file operations.

Example:

echo "hello" > /dev/ttyAMA10
cat /dev/ttyAMA10

Linux Device Model¶

Hardware devices are usually exposed in user space through device files.

Device files are located in:

/dev

Examples:

UART → /dev/tty*
SPI → /dev/spidev*
I2C → /dev/i2c-*
Storage → /dev/mmcblk*

These device files are handled by kernel drivers.

Summary¶

Linux architecture consists of three main layers:

User Space
    ↓
Kernel
    ↓
Hardware

User applications communicate with hardware through:

system calls
kernel drivers
device files