Break key

The Break key is a keyboard key found on many computer keyboards, historically used to interrupt or pause the current operation, often in command-line environments or early computing systems. Its function and behavior have evolved over time, and while less commonly used in modern graphical interfaces, it is still supported in certain legacy software, terminal emulators, and debugging tools.

History and origin

The Break key traces its roots to early teletype machines (such as the Teletype Model 33) and mainframe terminals, where a physical "break" signal was used to momentarily interrupt the transmission line, indicating that the remote system should stop sending data or respond to a control command. In these contexts, the Break key effectively sent a long space (or specific voltage level) that signaled a hardware interrupt. This mechanism was adopted by early personal computers: the IBM PC and its compatibles included a dedicated Break key on the keyboard, often combined with the Pause key on the same keycap. On the original IBM PC (1981), pressing Ctrl+Break would generate an interrupt (INT 1Bh) that could be caught by software to abort a running program or halt a listing. In MS-DOS and many text-mode applications, Ctrl+Break was a standard way to stop a batch file, terminate a command, or cancel output.

Function in various operating systems

MS-DOS and legacy environments

In MS-DOS, the Break key (typically used in combination with the Ctrl key) triggered a software interrupt that checked for a break condition. The command BREAK=ON in CONFIG.SYS allowed the operating system to check for break key presses during certain system calls, giving the user more responsive control over program termination. Standalone pressing of the Break key on early keyboards sometimes produced a hardware interrupt that could be captured by custom software, but in most DOS environments, the key was inactive unless combined with Ctrl or used within specific applications.

Windows

In Microsoft Windows, the behavior of the Break key depends on the context:

• In the Command Prompt (console), pressing Ctrl+Break (or sometimes just Break) can terminate the current foreground process, similar to Ctrl+C but with a different signal (SIGBREAK on Windows). In some versions, Ctrl+Break generates a hard termination that may not allow cleanup handlers to run.
• In graphical environments, the Pause/Break key when pressed alone often pauses output (e.g., during POST at system startup) or is mapped to system functions. On modern Windows keyboards, the key is often labeled "Pause/Break" and used with the Windows logo key (⊞ Win + Pause/Break) to open the System Properties dialog.
• In certain applications (e.g., debuggers, terminal emulators), the Break key sends a break signal over a serial or telnet connection, mimicking the original teletype function.

Unix and Linux

Unix-like systems typically do not have a dedicated Break key on standard PC keyboards. Instead, the key combination Ctrl+C (sending the INT signal) and Ctrl+\ (sending the QUIT signal) are used to interrupt or terminate processes. However, in terminal emulators connected to serial lines or in virtual consoles, the Break key can be mapped to generate a serial break condition. Some terminal programs (e.g., GTKterm, PuTTY) allow the user to send a break signal by pressing a menu option or a keyboard shortcut (often Ctrl+Break or Alt+Break). In the Linux kernel, the Break key on a PS/2 or USB keyboard is recognized as a separate scancode (0xE0 0x46, make; 0xE0 0xC6, break), but it is not assigned a default action by the kernel; user-space programs (like setkeycodes or input remapping tools) can assign it a function.

Other systems

On legacy systems such as CP/M, the Break key (often mapped to Ctrl+C) was used to exit a program or abort a command. In the IBM System/360 and mainframe terminals, the Break key sent an attention interrupt. On Apple keyboards (e.g., Apple II), the Break key was often labeled "RESET" and used in combination with Ctrl to perform a warm restart. Modern Mac keyboards do not include a Break key; however, some USB keyboards emulate it using Fn+Escape or alternative combinations.

Hardware and key placement

On most PC keyboards, the Break key is integrated with the Pause key. Pressing the key alone typically generates a Pause (temporarily halting scrolling output), while pressing Ctrl+Break or Alt+Break produces the break signal. In some extended keyboards, particularly those designed for terminal emulation or industrial use, there is a separate Break key. The key's scancode (on PS/2) is 0xE0 0x46 for the make and 0xE0 0xC6 for the break. On USB keyboards, it corresponds to the Usage ID 0x48 (Keyboard Pause) in the HID specification, which can be mapped by the operating system as either a pause or a break depending on driver and software handling.

Modern usage and alternatives

In contemporary computing, the Break key has largely been supplanted by other interrupt methods:

• In graphical user interfaces, program termination is typically done via window close buttons, task manager, or system dialogs.
• In command-line interfaces, Ctrl+C (SIGINT) is the standard way to interrupt a foreground process on Unix-like systems and in Windows (though Windows also uses Ctrl+Break for a more forceful termination in some contexts).
• For serial communication, the Break condition is often generated by holding the space line for a specified duration, which can be controlled via software or a dedicated break button in terminal emulators.

Nonetheless, the Break key remains present on many keyboards (especially full-size desktop models) for backward compatibility, and it is still used by power users in virtual machines, debuggers (like WinDbg), and legacy system management. Some programming environments (e.g., Borland Turbo C, early IDEs) recognized the Break key to abort compilation or break into the debugger.

See also

• Pause key
• Control character
• Interrupt
• Keyboard technology