Scroll Lock

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. The key is not frequently used, and therefore some reduced or specialized keyboards lack Scroll Lock altogether.

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

The Scroll Lock key is meant to lock all scrolling techniques and is a vestige of the original IBM PC keyboard. In its original design, Scroll Lock was intended to modify the behavior of the arrow keys. When the Scroll Lock mode is on, the arrow keys scroll the contents of a text window instead of moving the cursor. In this usage, Scroll Lock 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 Scroll Lock 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 Ctrl pressed.

Most GUI environments neglect Scroll Lock, 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, Ctrl+⇧ Shift 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 F7. 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.

Pressing the Scroll Lock 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 write 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 Scroll Lock 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, Ctrl+S (DC3 in ASCII) and Ctrl+Q (DC1 in ASCII) can be used instead of Scroll Lock in Unix-like systems to freeze and unfreeze the tty output respectively.