Code that uses x87 fpu and mmx instructions the

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Unformatted text preview: omputation The style of programming demonstrated in this example is supported by the floatingpoint instruction set. In cases where the stack structure causes computation bottlenecks, the FXCH (exchange x87 FPU register contents) instruction can be used to streamline a computation. Parameter Passing With the x87 FPU Register Stack Like the general-purpose registers, the contents of the x87 FPU data registers are unaffected by procedure calls, or in other words, the values are maintained across procedure boundaries. A calling procedure can thus use the x87 FPU data registers (as well as the procedure stack) for passing parameter between procedures. The called procedure can reference parameters passed through the register stack using the current stack register pointer (TOP) and the ST(0) and ST(i) nomenclature. It is also common practice for a called procedure to leave a return value or result in register ST(0) when returning execution to the calling procedure or program. When mixing MMX and x87 FPU instructions in the procedures or code sequences, the programmer is responsible for maintaining the integrity of parameters being passed in the x87 FPU data registers. If an MMX instruction is executed before the parameters in the x87 FPU data registers have been passed to another procedure, the parameters may be lost (see Section 9.5, "Compatibility with x87 FPU Architecture"). Vol. 1 8-5 PROGRAMMING WITH THE X87 FPU 8.1.3 x87 FPU Status Register The 16-bit x87 FPU status register (see Figure 8-4) indicates the current state of the x87 FPU. The flags in the x87 FPU status register include the FPU busy flag, top-ofstack (TOP) pointer, condition code flags, error summary status flag, stack fault flag, and exception flags. The x87 FPU sets the flags in this register to show the results of operations. FPU Busy Top of Stack Pointer 15 14 13 C B 3 11 10 9 8 7 6 5 4 3 2 1 0 C C C E S P U O Z D I 2 1 0 S F E E E E E E TOP Condition Code Error Summary Status Stack Fault Exception Flags Precision Underflow Overflow Zero Divide Denormalized Operand Invalid Operation Figure 8-4. x87 FPU Status Word The contents of the x87 FPU status register (referred to as the x87 FPU status word) can be stored in memory using the FSTSW/FNSTSW, FSTENV/FNSTENV, FSAVE/FNSAVE, and FXSAVE instructions. It can also be stored in the AX register of the integer unit, using the FSTSW/FNSTSW instructions. Top of Stack (TOP) Pointer A pointer to the x87 FPU data register that is currently at the top of the x87 FPU register stack is contained in bits 11 through 13 of the x87 FPU status word. This pointer, which is commonly referred to as TOP (for top-of-stack), is a binary value from 0 to 7. See Section 8.1.2, "x87 FPU Data Registers," for more information about the TOP pointer. Condition Code Flags The four condition code flags (C0 through C3) indicate the results of floating-point comparison and arithmetic operations. Table 8-1 summarizes the manner in wh...
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