It is a program on which application programs are executed

It acts as an interface between the user & the computer

Functions

• Security: OS uses password protection & other such techniques to protect user data
• Control over system performance: Monitors & records overall performance of the overall system to help in improving performance
• Job accounting: OS keeps track of time & resources used by various tasks & users
• Error detecting: OS detects errors, preventing from the malfunctioning of the computer system
• Coordination between software & user: OS coordinates & assigns compilers, interpreters and different other softwaes to various users of the computer system
• Memory management: OS anages different functionalities of primary memory or main memory, like, managing process allocation and requests, along with deadlock management
• Process management: In multi-programming, OS manages the order and access time of each process to access the processor
• Device management: OS manages differnt parts of a computer via their respective drivers
• File management: OS manages differnt files via a definite system, by keeping the files in different directories for efficient & easy usage

Process is an instance of a computer program that is being executed by one or many threads

Steps

Step 1: Whenever a NEW process is created, it is sent to the READY state.

Step 2: If there is no other process in the RUNNING state, the newly created process, which is in READY state, is sent to RUNNING state via some scheduler dispatch.

Step 3: If there is any higher priority process in the RUNNING state, the new process is sent to the WAITING state. Sometimes, some process waits for some input/output operations. After the completion of input/output operations, the process is sent to the READY state. In some cases, CPU temporarily suspends a RUNNING process, via some interrupt signal, to handle other higher priority tasks.

Step 4: After RUNNING of the process is complete, it is sent to TERMINATED state for execution.

OSI stands for Open System Interconnection

• Layer 1: Physical connection [fiber, wireless, hubs]
• Layer 2: Frames [ethernet, PPP (Point-to-Point Protocol), switch, bridge]
• Layer 3: Packets [IP (Internet Protocol), ICMP (Internet Control Message Protocol), IPSec (Internet Protocol Security), IGMP (Internet Group Management Protocol)]
• Layer 4: End-to-end connection [TCP (Transmission Control Protocol), UDP (User Diagram Protocol)]
• Layer 5: Sync & send to port [API, socket]
• Layer 6: Syntax [SSL (Secure Sockets Layer), JPEG (Joint Photographic Experts Group), MPEG (Moving Pictures Experts Group)]
• Layer 7: End User [HTTP (Hyper Text Transfer Protocol), DNS (Domain Name System)]

TCP/IP stands for Transmission Control Protocol/Internet Protocol

• Application Layer corresponds to layer numbers 5,6 & 7 of OSI Model
• Transport Layer corresponds to layer numbers 4 of OSI Model
• Internet Layer corresponds to layer numbers 3 of OSI Model
• Network Interface Layer corresponds to layer numbers 2 & 1 of OSI Model