Introduction to OS

Question 1

What is an OS?

Answer 1

The OS is a layer of software which manages some or all of the computers: disk, processor, memory and I/O devices

Question 2

Two main functions of an OS

Answer 2

To provide virtual machine abstraction of the underlying hardware.
To manage access to the CPU
A resource manager.

Question 3

OS as an Extended Machine

Answer 3

The OS provides usable abstractions for complex or difficult to use hardware.

Question 4

OS as a Resource Manager

Answer 4

It allows multiple programs to run at the same time, manages and protects memory, I/O devices and other resources.

It shares resources in time and in space.

Question 5

The OS kernel

Answer 5

The kernel is the core of the OS; it controls access to hardware and system resources. Kernel mode can be used for privileged operations.

Question 6

OS Structure

Answer 6

Many types of systems (e.g. mainframe, servers, mobiles, PCs) however one size does not necessarily fit all

OS needs to meet non functional requirements i.e. maintainable and reliable

Question 7

Monolithic OS

Answer 7

All OS layers are in Kernel (making the OS a single binary library linked to apps).

A bug in the kernel can bring the whole system down e.g. a bug in the audio driver can crash a whole system including unrelated applications.

Monolithic 0S (Linux): 15,000,000 lines of code

Question 8

Microkernel OS

Answer 8

For improved reliability Microkernel OS put as little as possible in the kernel (split into small, well-defined modules)

Runs only the essential modules in the kernel while drivers, filesystems etc are separate user mode processes.

MINIX3 microkernel: 15,000 lines of code