abend

(ABnormal END) An unexpected termination that causes the computer to stop responding. Also called a "crash," "bomb" or "lockup," an abend occurs when the computer is presented with instructions or data it cannot recognize, or a program tries to address memory beyond a defined boundary. Abends are generally the result of erroneous software logic in the application or operating system (see anomaly).

Bad Hardware Can Look Like Bad Software
A serious hardware failure will stop a computer that has no redundant components. For example, a short circuit on the motherboard will halt the operation; however, a failing memory cell can cause an instruction to point to an erroneous location, making it look like a software failure.

It Depends on the OS
If the abend occurs due to a bug in an application and the operating system is not crashproof, the computer locks up and has to be rebooted. Sophisticated operating systems attempt to halt only the offending application and allow the remaining applications to continue. As operating systems evolved through the years, they typically became more resilient to application bugs. There is less rebooting of the computer after an application crashes with newer versions of the OS than with older versions. The OS itself generally becomes more robust and crashes less often.

A Miracle It All Works
If you consider what goes on inside a computer, you might wonder why it does not crash more often. An ordinary home computer these days can easily have two billion memory cells. Every second, millions of them change their state from uncharged to charged and vice versa (0 to 1; 1 to 0). If only one cell fails, it can cause an instruction to be unrecognizable or an address to be invalid and results in an abend. See GPF and head crash.

Abending

Abending, or crashing, often occurs when the program points outside of its address space. This diagram depicts the anatomy of a program. "The Data" refers to constants used within the program and the input/output areas that hold the data while it is being processed. "The Processing" refers to the program's logic embodied in the flow chart and physically implemented as thousands of machine instructions (the columns).

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(jargon)

ABEND - /o'bend/, /*-bend'/ ABnormal END. Abnormal termination (of software); crash; lossage. Derives from an error message on the IBM 360; used jokingly by hackers but seriously mainly by code grinders. Usually capitalised, but may appear as "abend". Hackers will try to persuade you that ABEND is called "abend" because it is what system operators do to the computer late on Friday when they want to call it a day, and hence is from the German "Abend" = "Evening".