Inline assembler

Inline assembler is a feature of programming languages, that enables very low level code written in assembler to be embedded in a high level language like C.

This allows programmers to optimise a very performance-sensitive algorithm by sending individual commands to the computer's CPU.

This example of inline assembler is from the D programming language and computes the tangent of x using the x86's FPU instructions.

Example

// 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                 ; // 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                            ;
       jnp     Lret                    ; // C2 = 1 (x is out of range)
       // Do argument reduction to bring x into range
       fldpi                           ;
       fxch                            ;
SC17:  fprem1                          ;
       fstsw   AX                      ;
       sahf                            ;
       jp      SC17                    ;
       fstp    ST(1)                   ; // remove pi from stack
       jmp     SC18                    ;
   }
trigerr:
   return real.nan;
Lret:
   ;
}