Editing Inline assembler (section)

=== Processor-specific instruction in D ===
This example of inline assembly from the [[D programming language]] shows code that computes the tangent of x using the [[x86]]'s [[floating point unit|FPU]] ([[x87]]) instructions.

<syntaxhighlight lang="d">
// Compute the tangent of x
real tan(real x)
{
   asm
   {
       fld     x[EBP]                  ; // load x
       fxam                            ; // test for oddball values
       fstsw   AX                      ;
       sahf                            ;
       jc      trigerr                 ; // C0 = 1: x is NAN, infinity, or empty
                                         // 387's can handle denormals
SC18:  fptan                           ;
       fstp    ST(0)                   ; // dump X, which is always 1
       fstsw   AX                      ;
       sahf                            ; // if (!(fp_status & 0x20)) goto Lret
       jnp     Lret                    ; // C2 = 1: x is out of range, do argument reduction
       fldpi                           ; // load pi
       fxch                            ;
SC17:  fprem1                          ; // reminder (partial)
       fstsw   AX                      ;
       sahf                            ;
       jp      SC17                    ; // C2 = 1: partial reminder, need to loop
       fstp    ST(1)                   ; // remove pi from stack
       jmp     SC18                    ;
   }
trigerr:
   return real.nan;
Lret:                                    // No need to manually return anything as the value is already on FP stack
   ;
}
</syntaxhighlight>

The {{tt|fstsw-sahf}} followed by conditional jump idiom is used to access the x87 FPU status word bits C0 and C2. {{tt|fstsw}} stores the status in a general-purpose register; sahf sets the [[FLAGS register]] to the higher 8 bits of the register; and the jump is used to judge on whatever flag bit that happens to correspond to the FPU status bit.<ref>{{cite web |title=FSTSW/FNSTSW — Store x87 FPU Status Word |url=https://www.felixcloutier.com/x86/fstsw:fnstsw |quote=The FNSTSW AX form of the instruction is used primarily in conditional branching...}}</ref>