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Unformatted text preview: 1 in the x87 FPU status word (called in this situation the "round-up bit") is set if the significand was rounded upward and cleared if the result was rounded toward 0. After the result is stored, the OE flag is set and a software exception handler is invoked. The scaling bias value 24,576 is equal to 3 213. Biasing the exponent by 24,576 normally translates the number as nearly as possible to the middle of the double extended-precision floating-point exponent range so that, if desired, it can be used in subsequent scaled operations with less risk of causing further exceptions. When using the FSCALE instruction, massive overflow can occur, where the result is too large to be represented, even with a bias-adjusted exponent. Here, if overflow occurs again, after the result has been biased, a properly signed is stored in the destination operand. 8.5.5 Numeric Underflow Exception (#U) The x87 FPU detects a floating-point numeric underflow condition whenever the rounded result of an arithmetic instruction is tiny; that is, less than the smallest possible normalized, finite value that will fit into the floating-point format of the destination operand. (See Section 184.108.40.206, "Numeric Underflow Exception (#U)," for additional information about the numeric underflow exception.) Like numeric overflow, numeric underflow can occur on arithmetic operations where the result is stored in an x87 FPU data register. It can also occur on store floatingpoint operations (with the FST and FSTP instructions), where a within-range value in a data register is stored in memory in the smaller single-precision or double-precision floating-point formats. A numeric underflow exception cannot occur when storing values in an integer or BCD integer format, because a tiny value is always rounded to an integral value of 0 or 1, depending on the rounding mode in effect. Vol. 1 8-41 PROGRAMMING WITH THE X87 FPU The flag (UE) for the numeric-underflow exception is bit 4 of the x87 FPU status word, and the mask bit (UM) is bit 4 of the x87 FPU control word. When a numeric-underflow condition occurs and the exception is masked, the x87 FPU performs the operation described in Section 220.127.116.11, "Numeric Underflow Exception (#U)." When the exception is not masked, the action of the x87 FPU depends on whether the instruction is supposed to store the result in a memory location or on the x87 FPU resister stack. Destination is a memory location -- (Can occur only with a store instruction.) The UE flag is set and a software exception handler is invoked (see Section 8.7, "Handling x87 FPU Exceptions in Software"). The top-of-stack pointer (TOP) and source and destination operands remain unchanged, and no result is stored in memory. Because the data in the stack is in double extended-precision format, the exception handler has the option either of re-exchanges the store instruction after proper adjustment of the operand or of rounding the significand on the...
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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