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Unformatted text preview: 8-2). All addressing of the data registers is relative to the register on the top of the stack. The register number of the current top-of-stack register is stored in the TOP (stack TOP) field in the x87 FPU status word. Load operations decrement TOP by one and load a value into the new top-of-stack register, and store operations store the value from the current TOP register in memory and then increment TOP by one. (For the x87 FPU, a load operation is equivalent to a push and a store operation is equivalent to a pop.) Note that load and store operations are also available that do not push and pop the stack. Vol. 1 8-3 PROGRAMMING WITH THE X87 FPU FPU Data Register Stack 7 6 Growth Stack 5 4 3 2 1 0 ST(2) ST(1) ST(0) Top 011B Figure 8-2. x87 FPU Data Register Stack
If a load operation is performed when TOP is at 0, register wraparound occurs and the new value of TOP is set to 7. The floating-point stack-overflow exception indicates when wraparound might cause an unsaved value to be overwritten (see Section 220.127.116.11, "Stack Overflow or Underflow Exception (#IS)"). Many floating-point instructions have several addressing modes that permit the programmer to implicitly operate on the top of the stack, or to explicitly operate on specific registers relative to the TOP. Assemblers support these register addressing modes, using the expression ST(0), or simply ST, to represent the current stack top and ST(i) to specify the ith register from TOP in the stack (0 i 7). For example, if TOP contains 011B (register 3 is the top of the stack), the following instruction would add the contents of two registers in the stack (registers 3 and 5): FADD ST, ST(2); Figure 8-3 shows an example of how the stack structure of the x87 FPU registers and instructions are typically used to perform a series of computations. Here, a twodimensional dot product is computed, as follows: 1. The first instruction (FLD value1) decrements the stack register pointer (TOP) and loads the value 5.6 from memory into ST(0). The result of this operation is shown in snap-shot (a). 2. The second instruction multiplies the value in ST(0) by the value 2.4 from memory and stores the result in ST(0), shown in snap-shot (b). 3. The third instruction decrements TOP and loads the value 3.8 in ST(0). 4. The fourth instruction multiplies the value in ST(0) by the value 10.3 from memory and stores the result in ST(0), shown in snap-shot (c). 5. The fifth instruction adds the value and the value in ST(1) and stores the result in ST(0), shown in snap-shot (d). 8-4 Vol. 1 PROGRAMMING WITH THE X87 FPU Computation Dot Product = (5.6 x 2.4) + (3.8 x 10.3) Code: FLD value1 FMUL value2 FLD value3 FMUL value4 FADD ST(1) (a) R7 R6 R5 R4 R3 R2 R1 R0 5.6 ST(0) (b) R7 R6 R5 R4 R3 R2 R1 R0 13.44 ;(a) value1 = 5.6 ;(b) value2 = 2.4 ; value3 = 3.8 ;(c)value4 = 10.3 ;(d) (c) R7 R6 R5 ST(0) R4 R3 R2 R1 R0 13.44 39.14 ST(1) ST(0) (d) R7 R6 R5 R4 R3 R2 R1 R0 13.44 52.58 ST( ST Figure 8-3. Example x87 FPU Dot Product C...
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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