OS OVERVIEW

OPERATING SYSTEM OVERVIEW

1.DEFINITION:

• os is an interface between user and hardware of computer/system.
• os provides a platform for the users to run their programs on it
• os is similar like government, it performs no usefull functions by it self but it provides an environment with in which user can run his programs

2.BOOT STRAP LOADER:

• boot strap loader is a program present in RAM 
• this program loads OS present in Hard disc into RAM when system is turned on or restarted
• (HDD lo vunna OS ni RAM loki tisukuvache program ni BootStrapLoader program antaru)

3.MEMORY STRUCTURE:

• for any data to be processed it should under go following steps

1. data  is present in HDD
2. data is loaded into RAM from HDD
3. repeated instructions are loaded into CACHE
4. then the data is loaded into REGISTERS

• example if you wanna play a song then first the song is present in HDD (D drive), then it is loaded into RAM then from RAM to Registers then Song is processed ie., played 

4. DIRECT MEMORY ACCESS (DMA)

• If io devices like printers/scanners want to access some data for processing then for that data it request CPU.
• CPU which was busy with other works it has to resume that work and give data to io device
• we know in a system io requests are more, so for every io request CPU was interupted 
• so cpu gives DMA to io devices so by using DMA the io devices can directly access the data
• (IO DEVICES DATA KOSAM CPU NI INTERRUPT CHESTAI, SO CPU DMA DWARA DATA NI ACCESS CHESUKOMANI IO DEVICES KI DMA NI PROVIDE CHESTUNDI. INKA IO DEVICES CPU NI INTERRUPT CHEYAV DMA DWARA DATA NI ACCESS CHEUKUNTAI)

5.KERNAL:

• kernal is the core part of os 

user mode lo system ni shutdown down button click chestam,

 kernel mode lo system shutdown code execute ayyi system off avtundi

6.OPERATING SYSTEM SERVICES:

services provides by os to user are as follows

• user interface-(close,maximise,minimise,shutdown etc...)
• program execution-(play songs,movies,games---)
• IO operations-burning a dvd---
• file management-(create/edit/delete a file)
• communication-(by LAN)
• error detection-(usb removed,no paper in printer--)
• resource allocating
• accounting
• protection and security-(passwords)

7.SYSTEM CALLS:

• system call provides an interface to the services provided by operating system.
• operating system services system call dwara use chesukuntam
• ex: "security" is one of the service of OS it is used in UNIX by using system call "chmod"

8. VIRTUAL MACHINE:

vm provides virtual(not real) environment through which we can run more than one os on single hardware

9.PROCESS CONCEPTS:

• defination: A program which is under execution is know as process eg. a song which is playing is said to be a process
• process states: the state(position) of a process varies from time to time they are as follows

1. new-process is created
2. ready-process is ready to execute
3. running-process is in currently executing/running
4. waiting-process is waiting for anotherevent/request/resource
5. terminated-process is ended

• process states can be easily understood by above picture
• process control block: it maintains the information of the processes that are under execution

• context switching :it is the process in which CPU switches from one process to another process by saving state in pcb i know as conetxt switching

1. ex: assume there are two processes A and B, CPU is executing process A in that time only process B requests CPU to process it first, then CPU saves the state of A in PCB and switches to process B and executes process B,when process B is completed then CPU switches back to process A

• process scheduling: process scheduling is nothing but assigning a process to the CPU, it is done based on CPU scheduling algorithms.the cpu scheduling algorithms are as follows

1. FIRST COME FIRST SERVE : which process first into ready queue will be processed first
2. SHORTEST JOB FIRST: which process is short(small) that is processed first
3. PRIORITY SCHEDULING: which process has more priority that is processed first
4. ROUND ROBIN: each process is given some time interval in that time interval only that process will be executed

• IMPORTANT POINTS:

1. cpu utilization: how much time we have utilized cpu, we have to utilize cpu at maximum
2. Through put: number of processes that are completed by cpu in given time interval
3. Turnaround time: the time interval between submission of job and completion of the job
4. waiting time: sum of time spend by process during waiting in ready queue
5. response time: when was the first response taken place