6/05/2012

What is an Operating System?

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Operating System

a program that acts as an intermediate between a user and the computer hardware

controls all computers resource (memory, harddisk(s), ...)

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create a base for user level application so that user-level programmer dont have to worry about the hardware facility, e.g. in using printf.

Components inside Operating System

(Four Basic Components)

Process Management (Scheduler)

Creation and deletion of both user and system processes

Scheduling of the processes

Keep track the status of the processes

Provide channel for interprocess communication

Memory Management

Memory allocation and deallocation

Program memory space protection

Paging

Swapping

Virtual Memory

File System

Disk space management

Maintain directory structure

Enforce security

Control read and write access

Device Drivers, I/O

User Level vs. System Level

When a user-level program wants to get kernel services, it will issues a system call. (kernel call or supervisor call)

Next, the instruction will switch the process from user level to kernel level. (system level)

Here, the operating system will take over the control flow.

After examining the parameter given to the system call, the system call will be run.

Finally, the control is passed back to the user-level program and i.e. from kernel mode back to user level

You may look at /usr/src/linux/include/asm/unistd.h to see the listings of system calls

What is Kernel? Monolithic vs microkernel?

Generally, the operating system, as a running program, is called the kernel and have the four basic components as shown above.

A trend in modern OS is to implement more function in user level and leave a minimal kernel. (microkernel)

In Linux, all the components are kept inside kernel and so, it is not a microkernel structure. However, it is now moving to modules hierarchy, e.g. fs, mm, ...



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