Scroll Lock

Scroll Lock (⤓ or ⇳) is a lock key (typically with an associated status light) on most IBM-compatible computer keyboards. Depending on the operating system, it may be used for different purposes, and applications may assign functions to the key or change their behavior depending on its toggling state. The key is not frequently used, and therefore some reduced or specialized keyboards lack altogether.

Pressing performs the same function as pressing. This behavior is a remnant of the original IBM PC keyboards, which did not have a dedicated Break (Pause) key. Instead, they assigned the Pause function to and the Break function to.

Window scrolling
The key is meant to lock all scrolling techniques and is a vestige of the original IBM PC keyboard. In its original design, was intended to modify the behavior of the arrow keys. When the mode is on, the arrow keys scroll the contents of a text window instead of moving the cursor. In this usage, is a toggling lock key like Num Lock or Caps Lock, which have a state that persists after the key is released.

Today, this particular use of is rare. Modern programs honoring this behavior include Microsoft Excel, Microsoft Project, Image-Line FL Studio, IBM Lotus Notes, Forté Agent, Renoise, Cakewalk by BandLab, and LibreOffice Calc.

Some text editors (such as Notepad++, Microsoft Visual Studio) exhibit similar behavior when the arrow keys are used with Control key pressed.

Most GUI environments neglect, which means scrolling must be accomplished with a mouse, using means such as scrollbars or scroll wheels, or an "autoscroll" feature activated by the middle mouse button (which may be part of the scroll wheel). In some instances, + in conjunction with the arrow keys can enable scrolling.

Some web browsers including Google Chrome, Firefox, and Internet Explorer support a Caret browsing mode which can be toggled by. While the window scrolling behavior with Caret navigation disabled (the default) is similar to that suggested for Scroll Lock, enabling Caret navigation will enable a cursor scrolling behavior as if Scroll Lock were disabled.

Console scrolling
Pressing the key in the Linux console while the text is scrolling through the screen freezes the console output (but not input) during which no further text is sent to the screen, while the program continues running as usual, or become blocked at the write syscall when too much data prevented from reaching the terminal caused the tty's output queue to become full and the tty file descriptor is not using non-blocking I/O. When is pressed again, the screen is unfrozen and all text generated during the freeze is displayed at once and the program continues to run normally. This allows the user to pause the display and read long log messages that scroll through the screen too quickly to read, such as when the system is booting up (provided the keyboard driver has already been loaded). Unless configured otherwise or in raw mode, (DC3 in ASCII) and  (DC1 in ASCII) can be used instead of Scroll Lock in Unix-like systems to freeze and unfreeze the tty output respectively.

This behavior emulates the Hold Screen key or similar flow control mechanisms on computer terminal hardwares.

The behavior should be distinguished from pressing the Pause key during POST or under DOS, which, by default, has the effect of temporarily halting the running process.

On the FreeBSD system console, the key not only pauses output, but additionally enables scrolling: after pressing, one can use  and  to scroll up or down. Pressing again disables this mode and moves back to the bottom of the output. The Linux console implemented this behaviour by using and, not requiring  to be active; this behavior has been removed due to a vulnerability (CVE-2020-14390). The NetBSD console can be scrolled either with / or by pressing  then /.

Boot diagnostics
In Multiuser DOS, System Manager, and REAL/32, activating during boot will always enter a special diagnostic mode, where the operating system and loaded device drivers will display detailed status and debugging information about themselves and their interaction with the operating system, sometimes up to several screenfulls per loaded driver. Automatic scrolling is suspended for as long as remains activated.

DR-DOS 7.02 and higher provides a CONFIG.SYS directive named  to change the Scroll Lock status on the fly. Activating during boot will enter a DIAG mode as well, wherein some special drivers may display more verbose messages, but in contrast to the situation under the Multiuser DOS family normal DOS drivers do not typically display additional information. The operating system, however, will temporarily suspend any automatic timeouts in CONFIG.SYS trace  and / single-stepping modes for as long as  is active.

Debugging
In the Microsoft Windows NT family starting with Windows 2000, a debugging feature can be enabled, that allows the user to manually crash the system, generating a memory dump for analysis of the current state of the operating system. This can be accomplished by setting the REG_DWORD registry key  to. Once set, the user can hold the key and tap the  key twice to trigger a Blue Screen of Death.

In Microsoft Visual Basic 5.0 and 6.0, pauses the running program in the integrated development environment, as does its alias.

Copy and paste
In conjunction with the alternative DOS keyboard and console drivers FreeKEYB and K3PLUS, also serves as part of a copy & paste facility between applications. Compared to the standard keyboard driver KEYB these drivers offer several extensions including an extended keystroke buffer with key stacking facility, macro recorder, and a second cursor called CopyCursor, which can be invoked on demand (by default on pressing or the middle mouse button) and freely move on the screen using the cursor keys or the mouse, even outside the area reachable by the standard cursor in the running application. Once invoked, pressing (or the left mouse button) the characters under the CopyCursor can be stuffed, one after another, into the keyboard buffer, from where they will be read by the running application as emulated key (or Alt Numpad) input, thereby typically showing up at the location of the standard cursor. The CopyCursor will move to the next screen position after each (or backwards with each ). Normal keyboard input is possible while the CopyCursor is enabled, and the user can switch between the two cursors by toggling the hotkey again. Pressing or  exits the CopyCursor, so that, on its next invocation, it will show up at the position of the standard cursor again rather than at its previous location. If the screen contents are scrolled, the position of the CopyCursor will move accordingly until reaching the display limits. With activated, keypresses will still reach the normal keystroke buffer, while CopyCursor input will be stacked up internally for later use in a second queue, dynamically maintained within the extended keystroke buffer. Thereby, it is possible to "collect" selected screen output from different programs and spool out the data much later while within yet another application by toggling off again. It is also possible to use this as input into the macro recorder for later use as scrap macro. Since this is an integral part of these keyboard drivers, it is fully transparent to running software and therefore works with virtually any DOS program, including at the command prompt, temporary shelled programs, and task switchers.

Other uses
IBM PC documentation called an "inactive key". When PC Magazine asked an executive of keyboard manufacturer Key Tronic about the key's purpose in a 1983 interview, he replied "I don't know, but we put it on ours, too."

The indicator light is sometimes used for other purposes such as a keyboard layout indicator in some Linux distributions or other applications, because doing so is less likely to cause problems than manipulating other keys/lights.

Keyboard, video, mouse switches (KVM) often use the Scroll Lock key on the keyboard connected to the KVM switch for selecting between computers. On KVM switches with On-screen display (OSD), a "double click" of the Scroll Lock key often brings up the OSD, allowing the user to select the desired computer from a list or access the configuration menu of the KVM. Some models, especially those without an OSD, switch to the input corresponding to the number key pressed after Scroll Lock, or in the case of some two-port switches, the up and down arrow keys. Simple models with only two or four inputs may immediately switch to the next input following a double press of the Scroll Lock key. 
 * In FL Studio, the Scroll Lock key, when on, will prevent the screen from scrolling when the time marker moves out of the visible area.
 * On some Dell laptops the Scroll Lock key becomes the Fn key when an external keyboard is used. This behavior can be disabled in the BIOS.
 * In most 'LED light' Mods for keyboards, to give the keyboard an overhead LED so the keyboard can be lit up, the Scroll lock key is used to switch it On and Off and the LED light is unsoldered and a new brighter LED is attached overhead the keyboard.
 * In the Opera web browser the Scroll Lock key is utilized to toggle "listening mode" for the browser's built-in voice recognition feature.
 * In Synergy, the Scroll Lock key is utilized to lock the use of mouse and keyboard functions to a specific PC.
 * In Stepmania, pressing the Scroll Lock key opens the Options menu.
 * In the multilingual text editor iLeap, the Scroll Lock key is used as a toggle switch to change between two languages.
 * X Window System can be configured to indicate changing to an alternative keyboard layout by turning on the Scroll Lock light.
 * In FreeBSD, and other BSD descendants, Scroll Lock still functions similarly to IBM's original design.
 * Individual applications may assign any function to this key as an added convenience hotkey.
 * Xfire, a chat application for use while playing a video game, is activated by using the default key combination of "Scroll Lock" + "X". Xfire also uses Scroll Lock for other in-game purposes, such as taking screenshots. Sometimes its LED is used to notify users that an event occurred.
 * In the media system My Media System, navigation is done in a fashion very similar to the original idea of the Scroll Lock key. At the beginning and end of a list, navigating will move the cursor but not the view. In the middle of the list, the cursor will be locked in the middle of the screen while the view moves.
 * Konsole, a terminal emulator for KDE uses the Scroll Lock key to make the arrow keys scroll back the buffer, rather than going up and down in the command history (the default behavior of the arrow keys in Konsole). This has often led to problems while using the Xine media player, as Xine emits a Scroll Lock key signal to prevent the screensaver from starting. This will in turn make Konsole switch the arrow key behavior.
 * Maplestory, an online MMORPG, utilizes the Scroll Lock key to take pictures, ignoring its toggling function (that is, it does not matter if the Scroll Lock LED is on or off).
 * Combat Arms, a free-to-play microtransaction-based first-person shooter, uses the Scroll Lock key to lock the movement of chat messages.
 * TeamViewer, a program that allows users to watch and control another person's screen for use of technical support, uses the Scroll Lock key when viewing to send hotkeys directly to the person being viewed/controlled instead of the viewer's computer. For example, if the user wants to open task manager using CTRL+ALT+DEL on the computer or another user at that time, the scroll lock should be turned on.
 * Many programs (e.g. e-mail clients) will toggle the Scroll Lock and in the process flash the LED on and off to alert the user's attention.
 * Winamp uses scroll lock in its main visualizer, MilkDrop. When pressed, the visualizer preset is then locked and will not change until the scroll lock is pressed again.
 * The N64 Virtual Pad controller plugin for Project64 and 1964 (emulator) was the first to emulate the Nintendo 64 Rumble Pack, which it does so by flashing the Scroll Lock LED.

Scroll Lock on keyboards without a Scroll Lock key
Laptops and keyboards without a physical key may have a second function on another key that acts as if a  key is activated.

Some common methods are:
 * or on certain Dell laptops.
 * or on certain Lenovo laptops.
 * on certain HP laptops.
 * on Windows.

Another way to press is to use an on-screen keyboard. In Windows 8 and later versions, it can be found in the "Windows Ease of Access" program group. This will provide access to an emulation of a standard 101/102-key keyboard.