Kernel & User Mode


These are two different modes in which your computer can operate. Prior to this, when computers were like a big room, if something crashes – it halts the whole computer. So computer architects decide to change it. Modern microprocessors implement in hardware at least 2 different states.

  • User mode

Mode where all user programs execute. It does not have access to RAM and hardware. The reason for this is because if all programs ran in kernel mode, they would be able to overwrite each other’s memory. If it needs to access any of these features – it makes a call to the underlying API. Each process started by windows except of system process runs in user mode.


  • Kernel mode

Mode where all kernel programs execute (different drivers). It has access to every resource and underlying hardware. Any CPU instruction can be executed and every memory address can be accessed. This mode is reserved for drivers which operate on the lowest level


How the switch occurs.

The switch from user mode to kernel mode is not done automatically by CPU. CPU is interrupted by interrupts (timers, keyboard, I/O). When interrupt occurs, CPU stops executing the current running program, switch to kernel mode, executes interrupt handler. This handler saves the state of CPU, performs its operations, restore the state and returns to user mode.


The processes in an operating system must be protected from one another’s activities. To provide such protection, we can use various mechanisms to ensure that only processes that have gained proper authorization from the operating system can operate on the files, memory segments, CPU, and other resources of a system.

Protection refers to a mechanism for controlling the access of programs, processes, or users to the resources defined by a computer system. This mechanism must provide a means for specifying the controls to be imposed, together with a means of enforcement. We distinguish between protection and security, which is a measure of confidence that the integrity of a system and its data will be preserved.

What is the difference between security and protection?


Security is the degree of resistance to, or protection from, harm. It applies to any vulnerable and valuable asset, such as a person, dwelling, community, nation, or organization. Security provides “a form of protection where a separation is created between the assets and the threat.”

 Principles of Protection

  • The principle of least privilege dictates that programs, users, and systems be given just enough privileges to perform their tasks.
  • This ensures that failures do the least amount of harm and allow the least of harm to be done.
  • For example, if a program needs special privileges to perform a task, it is better to make it a SGID program with group ownership of “network” or “backup” or some other pseudo group, rather than SUID with root ownership. This limits the amount of damage that can occur if something goes wrong.
  • Typically each user is given their own account, and has only enough privilege to modify their own files.
  • The root account should not be used for normal day to day activities – The System Administrator should also have an ordinary account, and reserve use of the root account for only those tasks which need the root privileges.




                  Operating System Services

An Operating System provides services to both the users and to the programs.

  • It provides programs an environment to execute.
  • It provides users the services to execute the programs in a convenient manner.

Following are a few common services provided by an operating system.

  • Program execution
  • I/O operations
  • File System manipulation
  • Communication
  • Error Detection
  • Resource Allocation

1) Program execution

Operating systems handle many kinds of activities from user programs to system programs like printer spooler, name servers, file server, etc. Each of these activities is encapsulated as a process.

A process includes the complete execution context (code to execute, data to manipulate, registers, OS resources in use). Following are the major activities of an operating system with respect to program management.

  • Loads a program into memory.
  • Executes the program.
  • Handles program’s execution.
  • Provides a mechanism for process communication.
  • Provides a mechanism for deadlock handling.

2) I/O Operation

An I/O subsystem comprises of I/O devices and their corresponding driver software. Drivers hide the peculiarities of specific hardware devices from the users.

An Operating System manages the communication between user and device drivers.

  • I/O operation means read or write operation with any file or any specific I/O device.
  • Operating system provides the access to the required I/O device when required.

3) File system manipulation

A file represents a collection of related information. Computers can store files on the disk (secondary storage), for long-term storage purpose. Examples of storage media include magnetic tape, magnetic disk and optical disk drives like CD, DVD. Each of these media has its own properties like speed, capacity, data transfer rate and data access methods.

A file system is normally organized into directories for easy navigation and usage. These directories may contain files and other directions. Following are the major activities of an operating system with respect to file management.

  • Program needs to read a file or write a file.
  • The operating system gives the permission to the program for operation on file.
  • Permission varies from read-only, read-write, denied and so on.
  • Operating System provides an interface to the user to create/delete files.
  • Operating System provides an interface to the user to create/delete directories.
  • Operating System provides an interface to create the backup of file system.

4) Communication

In case of distributed systems which are a collection of processors that do not share memory, peripheral devices, or a clock, the operating system manages communications between all the processes. Multiple processes communicate with one another through communication lines in the network.

The OS handles routing and connection strategies, and the problems of contention and security. Following are the major activities of an operating system with respect to communication.

  • Two processes often require data to be transferred between them
  • Both the processes can be on one computer or on different computers, but are connected through a computer network.
  • Communication may be implemented by two methods, either by Shared Memory or by Message Passing.

5) Error handling

Errors can occur anytime and anywhere. An error may occur in CPU, in I/O devices or in the memory hardware. Following are the major activities of an operating system with respect to error handling.

  • The OS constantly checks for possible errors.
  • The OS takes an appropriate action to ensure correct and consistent computing.

6) Resource Management

In case of multi-user or multi-tasking environment, resources such as main memory, CPU cycles and files storage are to be allocated to each user or job. Following are the major activities of an operating system with respect to resource management −

  • The OS manages all kinds of resources using schedulers.
  • CPU scheduling algorithms are used for better utilization of CPU.



  • There are different kinds of operating systems: such as Windows, Linux and Mac OS
  • There are also different versions of these operating systems,g. Windows 7, 8 and 10
  • Operating systems can be used with different user interfaces (UI): text user interfaces (TUI) and graphical user interfaces (GUI) as examples
  • Graphical user interfaces have many similarities in different operating systems: such as the start menu, desktop etc.


When you can recognize the typical parts of each operating system’s user interface, you will mostly be able to use both Windows and Linux as well as e.g. Mac OS.


A user interface (UI) refers to the part of an operating system, program, or device that allows a user to enter and receive information. A text-based user interface displays text, and its commands are usually typed on a command line using a keyboard. With a graphical user interface the functions are carried out by clicking or moving buttons, icons and menus by means of a pointing device.

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