thumb|The scroll-lock key with an activated indicator light on an [[Model M keyboard|IBM Model M keyboard]]
Scroll Lock or ScrLk (sometimes notated ⤓ 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. 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 becomes blocked at the syscall when too much data is prevented from reaching the terminal, causing the tty's output queue to become full if 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 hardware.
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.
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