Break key
Encyclopedia
Computer keyboard
In computing, a keyboard is a typewriter-style keyboard, which uses an arrangement of buttons or keys, to act as mechanical levers or electronic switches...
go back to telegraph practices. A standard telegraph
Telegraphy
Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...
key has a built-in knife switch
Knife switch
A knife switch is a type of switch used to control the flow of electricity in a circuit. It is composed of a hinge which allows a metal lever, or knife, to be lifted from or inserted into a slot or jaw. The hinge and jaw are both fixed to an insulated base, and the knife has an insulated handle to...
that can be used to short
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
the key's contacts. When the key was not in use, that switch was kept closed, so that a signal was continually sent. If that steady-state signal was interrupted, it indicated one of two things: either the operator was about to start transmitting, or something else had happened to break the connection—such as a physical break in the telegraph line.
As the teleprinter
Teleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
came into use, a key was created that would temporarily interrupt the communications line, allowing this practice to continue. When this occurred, the receiving teleprinter mechanism would activate but not print characters (it was typing the non-printing DEL character repeatedly). The noise got the operator's attention.
Time-sharing
Time-sharing
Time-sharing is the sharing of a computing resource among many users by means of multiprogramming and multi-tasking. Its introduction in the 1960s, and emergence as the prominent model of computing in the 1970s, represents a major technological shift in the history of computing.By allowing a large...
computers connected to modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
s could recognize the break because the DEL characters received had "bad parity
Parity bit
A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....
." This was used to generate an interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
, allow login, or stop a running program.
On personal computers, the key can be used by software in several different ways, such as to switch between multiple login sessions, to terminate a program, or to interrupt a modem connection. On a BBC Micro
BBC Micro
The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation...
computer, the key generates an interrupt which would normally cause a warm restart of the computer. A cold restart is triggered by typing . If a DFS ROM is installed, will cause the computer to search for and load a file called
!boot on the floppy disk in drive 0.On the Sinclair ZX80
Sinclair ZX80
The Sinclair ZX80 is a home computer brought to market in 1980 by Science of Cambridge Ltd. . It is notable for being the first computer available in the United Kingdom for less than a hundred pounds...
and ZX81
Sinclair ZX81
The ZX81 was a home computer produced by Sinclair Research and manufactured in Scotland by Timex Corporation. It was launched in the United Kingdom in March 1981 as the successor to Sinclair's ZX80 and was designed to be a low-cost introduction to home computing for the general public...
computers, the Break is accessed by pressing Space. On the Sinclair ZX Spectrum it is accessed by and . The Spectrum+ and later computers have a dedicated key. It does not trigger an interrupt but will halt any running BASIC program, or terminate the loading or saving of data to cassette tape. An interrupted BASIC program can usually be resumed with the
CONTINUE command. The Sinclair QLSinclair QL
The Sinclair QL , was a personal computer launched by Sinclair Research in 1984, as the successor to the Sinclair ZX Spectrum...
computer, without a key, maps the function to .
On many modern PCs
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
, Pause interrupts screen output by BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
until another key is pressed. This is effective during boot
Booting
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
in text mode
Text mode
Text mode is a kind of computer display mode in which the content of the screen is internally represented in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of character cells, each of which contains one of the characters of a...
and in a DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
box in Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
safe mode with 50 lines. On early keyboards without a key (before the introduction of 101-key keyboards) the Pause function was assigned to , and the Break function to ; these key-combinations still work with most programs, even on modern PCs with modern keyboards. Pressing the dedicated key on 101-key keyboards sends the same scancode
Scancode
A scancode is the data that most computer keyboards send to a computer to report which keys have been pressed. A number, or sequence of numbers, is assigned to each key on the keyboard.- Variants :...
s as pressing , then , then releasing them in the reverse order would do; additionally, an e1hex prefix is sent which enables 101-key aware software to discern the two situations, while older software usually just ignores the prefix. The key is different from all other keys in that it sends no scancodes at all on release; therefore it is not possible for any software to determine whether this key is being held down.
On modern keyboards, the key is usually labeled Pause/Break. In most Windows environments, the key combination brings up the system properties. Compact and notebook keyboards often have the key mapped to a function command: for example .
Usage for breaking the program's execution
While both and combination are commonly implemented as a way of breaking the execution of a console application, they are also used for similar effect in integrated development environmentIntegrated development environment
An integrated development environment is a software application that provides comprehensive facilities to computer programmers for software development...
s. Although these two are often considered interchangeable, compilers and execution environments usually assign different signals to these. Additionally, in some kernels (e.g. miscellaneous DOS variants) is detected only at the time OS tries reading from a keyboard buffer and only if it's the only key sequence in the buffer, while is often translated instantly (e.g. by INT 1Bh under DOS). Because of this, is usually a more effective choice under these operating systems; sensitivity for these two combinations can be enhanced by the
BREAK=ON CONFIG.SYSCONFIG.SYS
CONFIG.SYS is the primary configuration file for the DOS, OS/2 as well as similar operating systems. It is a special file that contains setup or configuration instructions for the computer system.- Usage :...
statement.
See also
- SysRqSysRqSystem request is a key on keyboards for PCs that has no standard use. This key can be traced back to the operator interrupt key used on IBM 3270-type console keyboards of the IBM System/370 mainframe computer, which was used to cause the operating system such as VM/370 or MVS to allow the console...
- Scroll lockScroll lockScroll lock is a keyboard button on some computer keyboards. The key is not frequently used and therefore some keyboards lack Scroll Lock altogether....
- Num lockNum lockNum Lock is a key on the numeric keypad of most computer keyboards. It is a toggle key, like Caps Lock and Scroll Lock. Its state is commonly represented by an LED light built into the keyboard....