855 numeric underflow exception u and a software

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Unformatted text preview: -DOS compatibility mode. The mode of operation is selected by CR0.NE[bit 5]. (See Chapter 2, "System Architecture Overview," in the Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 3A, for more information about the NE flag.) 8.7.1 Native Mode The native mode for handling floating-point exceptions is selected by setting CR0.NE[bit 5] to 1. In this mode, if the x87 FPU detects an exception condition while executing a floating-point instruction and the exception is unmasked (the mask bit for the exception is cleared), the x87 FPU sets the flag for the exception and the ES flag in the x87 FPU status word. It then invokes the software exception handler through the floating-point-error exception (#MF, vector 16), immediately before execution of any of the following instructions in the processor's instruction stream: The next floating-point instruction, unless it is one of the non-waiting instructions (FNINIT, FNCLEX, FNSTSW, FNSTCW, FNSTENV, and FNSAVE). The next WAIT/FWAIT instruction. The next MMX instruction. If the next floating-point instruction in the instruction stream is a non-waiting instruction, the x87 FPU executes the instruction without invoking the software exception handler. 8.7.2 MS-DOS* Compatibility Sub-mode If CR0.NE[bit 5] is 0, the MS-DOS compatibility mode for handling floating-point exceptions is selected. In this mode, the software exception handler for floatingpoint exceptions is invoked externally using the processor's FERR#, INTR, and IGNNE# pins. This method of reporting floating-point errors and invoking an excep- Vol. 1 8-45 PROGRAMMING WITH THE X87 FPU tion handler is provided to support the floating-point exception handling mechanism used in PC systems that are running the MS-DOS or Windows* 95 operating system. The MS-DOS compatibility mode is typically used as follows to invoke the floatingpoint exception handler: 1. If the x87 FPU detects an unmasked floating-point exception, it sets the flag for the exception and the ES flag in the x87 FPU status word. 2. If the IGNNE# pin is deasserted, the x87 FPU then asserts the FERR# pin either immediately, or else delayed (deferred) until just before the execution of the next waiting floating-point instruction or MMX instruction. Whether the FERR# pin is asserted immediately or delayed depends on the type of processor, the instruction, and the type of exception. 3. If a preceding floating-point instruction has set the exception flag for an unmasked x87 FPU exception, the processor freezes just before executing the next WAIT instruction, waiting floating-point instruction, or MMX instruction. Whether the FERR# pin was asserted at the preceding floating-point instruction or is just now being asserted, the freezing of the processor assures that the x87 FPU exception handler will be invoked before the new floating-point (or MMX) instruction gets executed. 4. The FERR# pin is connected through external hardware to IRQ13 of a cascaded, programmable interru...
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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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