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OSdata.com: assembly language 

OSdata.com

Assembly Language

trap generating instructions

summary

    This web page examines trap generating instructions in assembly language. Specific examples of instructions from various processors are used to illustrate the general nature of assembly language.

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trap generating

    Trap generating instructions generate an exception that transfer control from software (usually application programs) to the operating system.

    Operating system traps provide a mechanism to change to higher privilege levels (if they exist on the processor) and usually include a mechanism for identifying what kind of trap has occurred. This allows application programs to make requests of the operating system and may provide a hardware mechanism for switching from a user mode to a superviser, kernel, or other higher privilege level for the operating system response to the request.

    A breakpoint instruction is used with external debugging hardware. The breakpoint instruction replaces an ordinary instruction (or the first part of an instruction) and relies on external debugging hardware to supply the missing instruction (or part of an instruction).

    Various check instructions will test for conditions, trapping if the test fails. One common example is a check against bounds or limits.

    Most processors have one or more illegal instructions. These are usually instructions that haven’t been implemented yet or instructions that have been dropped from a processor line. Many processors generate a trap or exception upon encountering an illegal instruction. Some processors will execute illegal instructions, which can lead to undocumented operations. Undocumented operations tend to change from one batch of processors to another and are highly unreliable. Some processors reserve an opcode that is guaranteed to always be illegal and always generate a trap or exception.

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    Programming example: I am making heavily documented and explained open source PHP/MySQL code for a method to play music for free — almost any song, no subscription fees, no download costs, no advertisements, all completely legal. This is done by building a front-end to YouTube (which checks the copyright permissions for you).

    View music player in action: www.musicinpublic.com/.

    Create your own copy from the original source code/ (presented for learning programming). Includes how to run this from your own computer if you don’t have a web site.


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    Copyright © 2000, 2001 Milo

    Created: February 21, 2001 (from machcode.htm)

    Last Updated: March 5, 2001


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