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Unformatted text preview: he exception handler to analyze or recover from the exception. To solve this problem, an exception synchronizing instruction (either a floating-point instruction or a WAIT/FWAIT instruction) can be placed immediately after any floating-point instruction that might present a situation where state information pertaining to a floating-point exception might be lost or corrupted. Floating-point instructions that store data in memory are prime candidates for synchronization. For example, the following three lines of code have the potential for exception synchronization problems: FILD COUNT INC COUNT FSQRT ;Floating-point instruction ;Integer instruction ;Subsequent floating-point instruction In this example, the INC instruction modifies the source operand of the floating-point instruction, FILD. If an exception is signaled during the execution of the FILD instruction, the INC instruction would be allowed to overwrite the value stored in the COUNT memory location before the floating-point exception handler is called. With the COUNT variable modified, the floating-point exception handler would not be able to recover from the error. Rearranging the instructions, as follows, so that the FSQRT instruction follows the FILD instruction, synchronizes floating-point exception handling and eliminates the possibility of the COUNT variable being overwritten before the floating-point exception handler is invoked. FILD COUNT FSQRT INC COUNT ;Floating-point instruction ;Subsequent floating-point instruction synchronizes ;any exceptions generated by the FILD instruction. ;Integer instruction The FSQRT instruction does not require any synchronization, because the results of this instruction are stored in the x87 FPU data registers and will remain there, undisturbed, until the next floating-point or WAIT/FWAIT instruction is executed. To abso- 8-44 Vol. 1 PROGRAMMING WITH THE X87 FPU lutely insure that any exceptions emanating from the FSQRT instruction are handled (for example, prior to a procedure call), a WAIT instruction can be placed directly after the FSQRT instruction. Note that some floating-point instructions (non-waiting instructions) do not check for pending unmasked exceptions (see Section 8.3.11, "x87 FPU Control Instructions"). They include the FNINIT, FNSTENV, FNSAVE, FNSTSW, FNSTCW, and FNCLEX instructions. When an FNINIT, FNSTENV, FNSAVE, or FNCLEX instruction is executed, all pending exceptions are essentially lost (either the x87 FPU status register is cleared or all exceptions are masked). The FNSTSW and FNSTCW instructions do not check for pending interrupts, but they do not modify the x87 FPU status and control registers. A subsequent "waiting" floating-point instruction can then handle any pending exceptions. 8.7 HANDLING X87 FPU EXCEPTIONS IN SOFTWARE The x87 FPU in Pentium and later IA-32 processors provides two different modes of operation for invoking a software exception handler for floating-point exceptions: native mode and MS...
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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.
- Winter '11