(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. With mature operating systems, there is less rebooting after an application crashes.

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 can easily have 16 billion memory cells. Every second, millions of them switch their status between charged and uncharged (1 to 0; 0 to 1). If only one cell fails, it can cause an instruction to be invalid, and an abend can occur. See GPF and head crash.