Crash (computing)

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A public payphone that has failed and is displaying the Blue Screen of Death.

A crash in computing is a condition where a program (either an application or part of the operating system) stops performing its expected function and also stops responding to other parts of the system. Often the offending program may simply appear to freeze. If this program is a critical part of the operating system kernel the entire computer may crash (a system crash).

Many crashes are the result of the execution of a single machine instruction, but the causes of this are manifold. Typical causes are when the program counter is set to an incorrect address or a buffer overflow overwrites a portion of program code due to an earlier bug. In either case, it is quite common for the processor to attempt to execute data or random memory values. Since all data values are possible but only some values are valid instructions, this often results in an illegal instruction exception. By chance such data or random values could be valid (though unplanned) instructions. If while successfully executing such instructions the processor happens onto a branch instruction that moves the program counter back a few bytes it would simply repeat those few instructions over and over, being stuck in an endless loop. One might say that the original bug that upset the program counter "caused" the crash, but the actual fault was an illegal instruction or endless loop some time later. The art of debugging such crashes is connecting the actual cause of the crash (easily determined) with the code that set off the chain of events. This is often very far from obvious—the original bug is usually perfectly valid code from the processor's perspective.

On earlier personal computers, it was actually possible to cause hardware damage through trying to write to hardware addresses outside of the system's main memory. Occasionally, execution of arbitrary data on a system will result in a breakup of screen display. This is widely considered a severe system crash.

Another cause of crashes is a race condition in communication between processes. One process may send a signal to a second process then stop execution until it receives a response. If the second process is busy the signal will be forced to wait until the process can get to it. However, if the second process was busy sending a signal to the first process then both processes would wait forever for the other to respond to signals and never see the other’s signal. If the processes are uninterruptible they will hang and have to be shut down. If at least one of the processes is a critical kernel process the whole system may crash and have to be restarted.

1 Etymology
2 Application crashes
3 Website server crashes
4 Operating system crashes
5 See also
6 External links

[edit] Etymology

The term "crash" may originate in the phrase "head crash" which would occur when the read/write heads in a hard disk would physically come into contact with (i.e. crash into) the disk surface. The result of this would usually be that the computer would freeze up.

[edit] Application crashes

An application typically crashes when it performs an operation which is not allowed by the operating system. The operating system then shuts down the application.

Typical errors that result in application crashes include:

attempting to read or write memory that is not allocated for reading or writing by that application (general protection fault)
attempting to execute privileged or invalid instructions
unforeseen circumstances or poor code writing that results in the program executing an endless loop
attempting to perform I/O operations on hardware devices to which it does not have permission to access
passing invalid arguments to system calls
attempting to access other system resources to which the application does not have permission to access (bus error)

[edit] Website server crashes

The software running the server behind a website may crash, rendering it inaccessible entirely or providing only an error message instead of normal content.

[edit] Operating system crashes

An operating system crash often happens when a hardware exception occurs which cannot be handled, such as a hardware exception occurring within the operating system itself.

Operating system crashes can also occur when internal sanity-checking logic within the operating system detects that the operating system has lost its internal self-consistency.

In an ideal world, well-written operating systems should not be able to be crashed by application-level activity. However, until 1993, with the release of Windows NT 3.1, this wasn't the case for the average PC. Industrial strength systems have enjoyed this sort of stability for much longer.

UNIX based operating systems tend to crash less often than those that are derived from other sources. Some examples of UNIX based systems include Mac OS X, GNU/Linux, BSD, and NEXTSTEP.

Recent versions of Microsoft Windows are more stable than its earlier versions. Windows XP is a 32-bit operating system, having versions for both home and business users. Previously the only available Windows operating systems designed for home use were hybrid 16-bit and 32-bit, like Windows 95, Windows 98/98SE and Windows ME, which didn't have the stability of a self-protecting operating system. Some home users would however run Windows 2000 Professional, but it was primarily targeted at the business market. Since the introduction of Windows XP Home, home users have enjoyed a much more stable operating system.