Single user mode is a mode in which a multiuser computer operating system boots into a single superuser. It is mainly used for maintenance of multi-user environments such as network servers. Some tasks may require exclusive access to shared resources, for example running fsck on a network share. This mode can also be used for security purposes - network services are not run, eliminating the possibility of outside interference. On some systems a lost superuser password can be changed by switching to single user mode, but not asking for the password in such circumstances is viewed as a security vulnerability.
Mac OS X
Mac OS X users can accomplish this by holding down Command-S after powering the system. The user may be required to enter a password set in the firmware. Single User Mode is different from a Safe Mode boot in that the system goes directly to the console instead of starting up the core elements of Mac OS X (items in /System/Library/, ignoring /Library/, ~/Library/, et al.). From there users are encouraged by a prompt to run fsck or other command line utilities as needed (or installed
Microsoft Windows
Microsoft Windows provides Recovery Console, Last Known Good Configuration, Safe Mode and recently Windows Recovery Environment as standard recovery means. Also, bootable BartPE-based third-party recovery discs are available.
Recovery Console and recovery discs are different from single user modes in other operating systems because they are independent of the maintained operating system.
Unix family
Unix-like operating systems provide single user mode functionality either through the SystemV style runlevels or to the BSD style boot-loader options.
Run levels are usually changed using the init command, runlevel 1 or S will boot into single user mode.
Boot-loader options can be changed during startup before the execution of the kernel. On FreeBSD and DragonFly BSD it can be changed before rebooting the system with the command nextboot -o "-s" -k kernel. On Sun Solaris it can be changed with the command reboot -- -s.
Computer Buses A computer system consists of different devices.CPU must be able to communicate with all devices. The devices are connected together by a communications channel called bus. A bus consists of a set of communication lines or wires. It is used to move a large amount of bits in the form of electrical pulses from one unit to another. The bus is used to connect the following units: Central Processing Unit Control Unit Arithmetic and Logic Unit Main Memory ( RAM, ROM) Input / Output Devices Bus is a common path to transfer data and commands between CPU, memory and input / output devices.It is also used to send or receive data from secondary storage.The capacity of a bus depends on the number of data lines in it.A bus with 16 lines can carry 16 bits or 2 bytes at a time. A bus with 32 lines can carry 32 bits or 4 bytes at a time . Types Of Buses Different types of buses are as follows: 1. Data Bus 2.Addres...
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