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Kernel mode vs user mode in linux
Kernel Mode Vs User Mode
Kernel Mode and User Mode
What is the Kernal?
• The kernel is the "core" of any computer system.
• It is the "software" which allows users to share
• The kernel can be thought as the main software
of the OS (Operating System), which may also
include graphics management.
• For example, under Linux (like other Unix-like
OSs), the XWindow environment doesn't belong to
the Linux Kernel, because it manages only graphical
operations (it uses user mode I/O to access video
• By contrast, Windows environments (Win9x,
WinME, WinNT, Win2K, WinXP, and so on) are a
mix between a graphical environment and kernel.
• Interacts with the hardware
• First program to get loaded when the system starts and runs till the
session gets terminated
• Different from BIOS which is hardware dependent.
• Kernel is software dependent
• LINUX: In hard disk, it is represented by the file /vmlinuz.
• Resource Management
• Xwindow- Graphical User Interface
• Sudo followed by command name – Super user do – file
extraction not allowed inside the file system.
o All OS related code are stuffed in a single module
o Available as a single file
o Advantage : Faster functioning
o OS components are isolated and run in their own address space
o Device drivers, programs and system services run outside kernel
memory space.Only a few functions such as process scheduling,
and interprocess communication are included into the
o Supports modularity & Lesser in size
Kernel Mode(Privileged Mode)
• Kernel mode, also referred to as system mode.
• two distinct modes of operation of the CPU
(central processing unit) in Linux
o Kernal mode and user mode.
User mode -non-privileged mode for user
Kernel Mode - Mainly for Restriction/
Protection from unauthorized user application
• When the CPU is in kernel mode, it is assumed to be executing
trusted software, and thus it can execute any instructions and
reference any memory addresses (i.e., locations in memory).
• all other programs(user applications) are considered untrusted
• Thus, all user mode software must request use of the kernel by
means of a system callin order to perform privileged
instructions, such as process creation or input/output
• A good example of this would be device drivers.
• A device driver must tell the kernel exactly how to
interact with a piece of hardware, so it must be run in
• Because of this close interaction with the kernel, the
kernel is also a lot more vulnerable to programs
running in this mode, so it becomes highly crucial
that drivers are properly debugged before being
released to the public.
A system call is a request to the kernel in a Unix operating system by
an active process for a service performed by the kernel.
• A process is an executing instance of a program. An active process is
a process that is currently advancing in the CPU (while other
processes are waiting in memory for their turns to use the CPU).
• Input/output (I/O) is any program, operation or device that transfers
data to or from the CPU and to or from a peripheral device (such as
disk drives, keyboards, mice and printers).
It is a non-privileged mode in which each process (i.e., a running instance of
a program) starts out.
It is non-privileged in that it is forbidden for processes in this mode to access
those portions of memory (i.e., RAM) that have been allocated to the kernel
or to other programs.
When a user mode process (i.e., a process currently in user mode) wants to
use a service that is provided by the kernel (i.e., access system resources
other than the limited memory space that is allocated to the user program), it
must switch temporarily into kernel mode, which has root (i.e.,
administrative) privileges, including root access permissions (i.e.,
permission to access any memory space or other resources on the system).
When the kernel has satisfied the process's request, it restores the process to
The standard procedure to switch from user mode to kernel mode is to call
the 0x80 software interrupt.
USER MODE(non –privileged Mode)
• User mode is the normal mode of operating for programs. Web
browsers, calculators, etc. will all be in user mode.
• They don't interact directly with the kernel, instead, they just give
instructions on what needs to be done, and the kernel takes care of the
• Code running in user mode must delegate to system APIs to access
hardware or memory.
• Due to the protection afforded by this sort of isolation, crashes in user
mode are always recoverable.
• Most of the code running on your computer will execute in user mode.
• When in User Mode, some parts of RAM cannot be addressed, some
instructions can’t be executed, and I/O ports can’t be accessed
What is the difference between User Mode and
when computers were as big as a room, users ran their
applications with much difficulty and, sometimes, their
applications crashed the computer.
To avoid having applications that constantly crashed, newer
OSs were designed with 2 different operative modes:
Kernel Mode: the machine operates with critical data
structure, direct hardware (IN/OUT or memory
mapped), direct memory, IRQ, DMA, and so on.
User Mode: users can run applications.
Switching from User Mode to Kernel Mode
When do we switch?
Once we understand that there are 2 different modes, we
have to know when we switch from one to the other.
Typically, there are 2 points of switching:
When calling a System Call: after calling a System Call,
the task voluntary calls pieces of code living in Kernel
When an IRQ (or exception) comes: after the IRQ an IRQ
handler (or exception handler) is called, then control
returns back to the task that was interrupted like nothing
SWITCHING FROM USER MODE TO KERNEL MODE
The only way an user space application can explicitly initiate a switch to kernel
mode during normal operation is by making an system call such as open,
read, write etc.
Whenever a user application calls these system call APIs with appropriate
parameters, a software interrupt/exception(SWI) is triggered.
As a result of this SWI, the control of the code execution jumps from the
user application to a predefined location in the Interrupt Vector
Table [IVT] provided by the OS.
This IVT contains an address for the SWI exception handler routine,
which performs all the necessary steps required to switch the user application to
kernel mode and start executing kernel instructions on behalf of user process.
Switch User Mode to Kernel Mode
IVT do the
• An interrupt is a signal to the operating system that an
event has occurred, and it results in changes in the
sequence of instructions that is executed by the CPU. In
the case of a hardware interrupt, the signal originates
from a hardware device such as a keyboard (e.g., when a
user presses a key), mouse or system clock (a circuit that
generates pulses at precise intervals that are used to
interrupt is an interrupt that originates in software,
usually by a program in user mode.
Kernel Mode "prevents" User Mode applications from
damaging the system or its features.
Modern microprocessors implement in hardware at least 2
different states. For example under Intel, 4 states
determine the PL (Privilege Level). It is possible to use
0,1,2,3 states, with 0 used in Kernel Mode.
Unix OS requires only 2 privilege levels, and we will use
such a paradigm as point of reference.
System calls are like special functions that manage OS
routines which live in Kernel Mode.
A system call can be called when we:
access an I/O device or a file (like read or write)
need to access privileged information (like pid(process
identifier, changing scheduling policy or other
need to change execution context (like forking or executing
some other application)
need to execute a particular command (like ''chdir'', ''kill",
''brk'', or ''signal'')
Program that interacts with kernel
Bridge between kernel and the user
User can type command and the command is conveyed
to the kernel and it will be executed
Types of Shell
Sh – simple shell
BASH – Bourne Again Shell
KSH – Korne Shell
CSH – C Shell
SSH – Secure Shell
To use a particular shell type the shell name at the command prompt.
Eg $csh – will switch the current shell to c shell
To view the current shell that is being used, type echo $SHELL at the command