1 - Introduction to OS

What is an OS

Definition. An OS is a program that is an intermediary between a user and computer hardware.

OS Type	Usage	Advantages	Disadvantages
No OS	Changing physical config	- minimal overhead	not portable, not efficient use
Batch OS	(Mainframes) similar jobs are bundled with instructions that OS execs in order.	- computer does more work	- no protection - infinite loops - hard to debug - no concurrency
Time-sharing OS	Multiple users interact with machine via terminal. job scheduling mimics concurrency.	- memory management - shares CPU time, memory, storage. - Virtualization
PC OS – Windows	Single user at once, but multiple user access	(dedicated machine)
PC OS – Unix	One user at workstation, but others can remote access.	(general time sharing model)

OS is an abstraction.

Hardware components have same core functionality but different attributes (speed, storage etc.)
Virtualization: each program executes as if it has all the resources to itself (e.g. memory space, files as contiguous entities when they are actually disjoint, illusion of process always running)

OS is a resource allocator. Manage resources and arbitrate conflicting requests.

OS is a control program. Prevents accidents and malicious attacks, provides security isolation and protection.

Kernel: complete access to hardware resources; User: partial access.

Kernel: Deals with hardware issues, provides system call interface, special code for interrupt handlers, device drivers.

In C/C++ system calls can be made via the library version (D --> C --> A)

A: OS exec machine instructions
B: Normal exec machine instructions
C: System calls (interface)
- D: Program calls library code
- In C/C++ system calls can be made via the library version (D -> C -> A)

Code organization:

OS Structures:

	Monolithic	Microkernel
Size	Bigger	Smaller
Execution	Fast	Slow
Extensibility	Highly coupled, difficult to extend	Ease of extensibility
Security (e.g. a service crashing)	Highly coupled, may cause whole system to crash	Causes only that microkernel to crash
Examples	Unix/Win/BSD	QNX, Symbian, L4Linux

OS assumes total control of hardware. So what is we want to run multiple OS on the same hardware? (e.g. Cloud computing)

Software emulation of hardware = Virtualization of underlying hardware (Illusion of hardware access)

Hypervisor/VMM

Type 1: Runs directly on hardware.
- Provides individual virtual machines to multiple guest OSes
Type 2: Runs as a program on a Host OS