A CDE Definition
(ABnormal END) Pronounced "ahb-end." An unexpected termination that causes the computer to stop responding. An abend occurs either 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).
Crash, Freeze, Lock Up and Hang
All the terms above refer to a program coming to an abnormal end; however, a "crash" occurs when the computer issues a "fault" and deliberately halts that line of execution. The terms "freeze," "lock up" or "hang" may refer to software that is actually still running but has erroneously wound up in an endless, internal loop that renders the program useless. In practice, the terms "crash," "freeze," "lock up" and "hang" are used synonymously. See infinite loop.
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 resilient, the computer locks up and has to be rebooted. Modern operating systems attempt to halt only the offending application and allow the remaining applications to continue. As operating systems evolve through the years, they become more bug-free themselves and more tolerant of application bugs, and there is less rebooting when an application stops working. However, all operating systems are not 100% foolproof, and bad applications do cause operating systems to crash; a major motivation for virtualizing computers (see virtualization and virtual machine).
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 32 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 head crash, GPF, active area and transistor concept.
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