Operating system (OS)

# Metaphor

The OS is a lot like a toyshop manager. They are required to:

• Direct the operational resources
  • Controls the use of employees’ time,
  • Distribute tools and parts between workers
• Enforces working policies
  • Ensures fairness between workers,
  • Make sure toys are made safely,
  • How to clean up after the job has been completed.
• Mitigates difficulty of complex tasks
  • Simplifies operations for each worker to do,
  • Chooses optimal task allocation to improve performance.

In comparison an OS does the following:

• Direct the operational resources
  • Controls the use of CPU and memory,
  • Controls the use of peripheral devices.
• Enforces working policies
  • Can ensure fair use of the resources between different processes,
  • Limit resources a certain process has access to.
• Mitigates difficulty of complex tasks
  • Abstracts hardware (system calls).

# Examples

• Desktop
  • Microsoft Windows
  • UNIX-based
    • Mac OS X (BSD)
    • Linux
• Embedded
  • Android
    • A form of Linux
  • iOS
  • Symbian

We will focus on Linux in this course.

# OS elements

There are 3 main OS elements:

Abstractions:

• process,
• thread,
• file,
• socket, and
• memory page.

Mechanisms:

• create,
• schedule,
• open,
• write, and
• allocate.

Policies:

• Least-recently used (LRU) ,
• Earliest deadline first (EDF)

# Example: Memory management

# Design principles

• Separation of mechanism and policy
  • Implement flexible mechanisms that support many policies
  • In different settings different policies make more sense.
• Optimise for common case
  • Where will the OS be used?
  • What will the user want to execute on that machine?
  • What are the workload requirements?

# User/Kernel protection

Process modes

Trap instruction

User mode applications have to access hardware through system calls. The OS can notify applications through signals.

# System call flow chart

System call

# OS architecture

At first OS included all the features within one monolithic application. This had the following advantages:

• You have everything already, you do not need to go somewhere else, and
• You can use compile time optimizations to improve efficiency. However, it had the following downsides:
• Lower customisation, portability and manageability,
• Higher memory footprint to run the OS, and
• Lower performance when you do not need all aspects of the OS.

OS such as Linux instead have gone for a modular approach, where OS applications have to conform to a standard interface for system calls. The user then can add the modules to the OS that they want to use. This had the following advantages:

• Easier to maintain as it is less code,
• Smaller footprint in memory,
• Less resources used to run the OS , However, it comes with the following downsides:
• Indirection can affect performance,
• Maintenance can be an issue as you rely on lots of different code bases that can introduce bugs.

For embedded devices another architecture is common called a microkernel. These typically only handle memory and process management. However, it adds a standard inter-process communication call, as services such as file systems or disk drivers are now application level processes rather than handled by the OS. This has the following advantages:

• They are normally a very small code base.
• It is easy to verify behaviour, so you can be sure the OS behaves well. However, they come with a lot of downsides:
• They are not normally portable as they are designed for specific hardware.
• Software development is more complex as they need to interact with different processes that normally would be part of the OS .
• There is lots of user/kernel crossing as they need to use the IPC a lot.

# Linux

The Linux OS is built in a layer architecture to make it simpler to use.

The kernel itself has a couple of components that can all be switched out for specific users needs.

# Mac

Mac uses a microkernel for all low-level operations, with a BSD component that provides a UNIX interface for the rest of the OS.