OS

Concept

Operating system

• aka. kernels
• the lowest level software, runs in the kernel mode
• program that acts as an intermediary between the user and the computer hardware
• other programs are run in the user mode

Abstraction

• programs interact with OS
• OS handles specifics of the low level, ie. drivers
• simplifies programming
• makes programs more portable

Resource allocation

• multiple programs can be run simultaneously
• OS manages CPU, memory and IO
• ensure efficient and fair resource allocation

Control

• programs might misuse the computer
  • accidentally: bugs
  • maliciously: virus/malware
• multiple users might share the computer
• OS prevents errors handles access control

OS structures

Monolithic

• OS is one big program
• services are packaged in the OS
• traditional approach, good performance but complex internal structure

Microkernel

• OS only provides minimal services
  • inter-process communication(IPC), address space management and thread management
• higher level programs need to use IPC to communicate
• more robust and better protection for the kernel but lower performance

Layered

• similar to monolithic
• OS is a hierachy of layers, lowest is hardware, highest is UI

Client-server

• variant of microkernel
• client process request service from server process
• server process runs on microkernel
• client and server can be on different machines

Virtual machines

• software emulation of hardware
• hypervisor runs VMs and monitors them

Type 1 hypervisor

• runs directly on computer hardware

Type 2 hypervisor

• runs as a program on the host OS