• The Linux operating system is composed of four major subsystems:

1. User Applications — examples include a word-processing application and a web-browser.
2. O/S Services — considered part of the operating system; also, the programming interface to the kernel is included.
3. Linux Kernel — abstracts and mediates access to the hardware resources.
4. Hardware Controllers —  for example, the CPU, memory hardware, hard disks, etc.

• Each subsystem layer can only communicate with the subsystem layers that are immediately adjacent to it.
• The dependencies between subsystems are from the top down.

Kernel

• presents a virtual machine interface to user processes.
  
• composed of five main subsystems

1. Process Scheduler
2. Memory Manager
3. Virtual File System
4. Network Interface
5. Inter-Process Communication

System Data Structures

1. Task List
2. Memory Map
3. I-nodes
4. Data Connection