Also found in: Dictionary, Thesaurus, Medical, Financial, Acronyms, Idioms, Wikipedia.
Controlling the frequency of an oscillator by means of an applied signal of constant frequency.
Automatic following of a target by a radar antenna.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
abend(ABnormal END) Pronounced "ab-end." An abend is an unexpected termination that causes the computer, smartphone or tablet to stop responding. The abend occurs either when the processor 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
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-based device 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 are more tolerant of application bugs. However, operating systems themselves are not 100% foolproof, and bad applications can cause operating systems to crash; a major motivation for running in a virtual mode (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 64 billion memory (RAM) 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.
|The green blocks are machine instructions executed by the CPU one after the other until a branch (jump) instruction breaks the sequence and points to an instruction elsewhere in the program. Abending (crashing, hanging, etc.) occurs when the program erroneously points outside of its address space, typically due to bad logic.|
file and record lockingA first-come, first-served technique for managing data in a multiuser environment. The first user to access the file or data record prevents, or locks out, other users from accessing it. After the file or record is updated, it is unlocked and available. File and record locking is an essential part of every database management system (DBMS), document management system and any other system that allows data to be updated by multiple users or applications. See also lock down.
lock downTo restrict the functionality of a system. If network administrators lock down client desktops, it means that users can perform only certain operations. For example, they might not be able to install new applications. The term is also used to configure a computer or service to prevent unwanted intrusions such as viruses or spam. In such cases, the system may be said to be "locked down."
Lock Down vs. Lock Up
Lock down deals with restrictions, whereas "lock up" refers to not working (see abend).
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.