At the start of each numeric instruction to describe

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Unformatted text preview: "prologue" and the body of the handler must not contain any floating-point instructions that cannot complete execution when there is a pending floating-point exception. (The no-wait instructions are discussed in Section 8.3.12, "Waiting vs. Non-waiting Instructions.") Note that the handler must still clear the exception flag(s) before executing the IRET. If the exception handler uses neither of these techniques, the system will be caught in an endless loop of nested floating-point exceptions, and hang. The body of the exception handler examines the diagnostic information and makes a response that is necessarily application-dependent. This response may range from halting execution, to displaying a message, to attempting to repair the problem and proceed with normal execution. The epilogue essentially reverses the actions of the prologue, restoring the processor so that normal execution can be resumed. The D-18 Vol. 1 GUIDELINES FOR WRITING X87 FPU EXCEPTION HANDLERS epilogue must not load an unmasked exception flag into the x87 FPU or another exception will be requested immediately. The following code examples show the ASM386/486 coding of three skeleton exception handlers, with the save spaces given as correct for 32-bit protected mode. They show how prologues and epilogues can be written for various situations, but the application-dependent exception handling body is just indicated by comments showing where it should be placed. The first two are very similar; their only substantial difference is their choice of instructions to save and restore the x87 FPU. The trade-off here is between the increased diagnostic information provided by FNSAVE and the faster execution of FNSTENV. (Also, after saving the original contents, FNSAVE re-initializes the x87 FPU, while FNSTENV only masks all x87 FPU exceptions.) For applications that are sensitive to interrupt latency or that do not need to examine register contents, FNSTENV reduces the duration of the "critical region," during which the processor does not recognize another interrupt request. (See the Section 8.1.10, "Saving the x87 FPU's State with FSTENV/FNSTENV and FSAVE/FNSAVE," for a complete description of the x87 FPU save image.) If the processor supports Streaming SIMD Extensions and the operating system supports it, the FXSAVE instruction should be used instead of FNSAVE. If the FXSAVE instruction is used, the save area should be increased to 512 bytes and aligned to 16 bytes to save the entire state. These steps will ensure that the complete context is saved. After the exception handler body, the epilogues prepare the processor to resume execution from the point of interruption (for example, the instruction following the one that generated the unmasked exception). Notice that the exception flags in the memory image that is loaded into the x87 FPU are cleared to zero prior to reloading (in fact, in these examples, the entire status word image is cleared). Example D-3 and Ex...
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This note was uploaded on 10/01/2013 for the course CPE 103 taught by Professor Watlins during the Winter '11 term at Mississippi State.

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