illegal operation

An illegal operation, a term often seen in error messages, is a command to the operating system or an instruction to the computer processor that specifies an operation that is unknown to the operating system or processor and that therefore can't be performed. The result is that the operating system or processor usually terminates the program that made the "illegal" request. If that program was an application that you were using, you may find that data you were entering was lost or, in some cases, can be recovered from a backed-up copy that the system made for you.

A computer's processor knows what operation to perform as the result of an operation code (or opcode) within the instruction. Failing to recognize an opcode, the processor will return information to the operating system which in turn will usually terminate the application program that sent the bad instruction and issue a message to the user about the illegal operation. An illegal operation may mean a bug in the application program (or occasionally in the operating system) or it may be the result of a combination of unusual circumstances that the program and operating system could or did not anticipate. In the latter case, the problem may not be repeated or may be intermittent.

The usual practice if you get an illegal operation message is to continue working if the application is still running, or to restart the program if it has been terminated, and, if the problem recurs, to reboot the operating system and try again. If you continue to experience illegal operation errors with an application, report the problem to the software vendor.

This was last updated in September 2005

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