Inline assembler

In computer programming, an inline assembler is a feature of some compilers that allows low-level code written in assembly language to be embedded within the existing program code that would otherwise have been created using a high-level language such as C or Ada. This embedding is usually done for one of three reasons:

The ISO C+ standard and ISO C standards (annex J) specify a conditionally supported syntax for inline assembler:

    An asm declaration has the form
    asm-definition:
    asm ( string-literal ) ;
    The asm declaration is conditionally-supported; its meaning is implementation- defined.

Calling an operating system directly is generally not possible in the presence of protected memory. The OS runs at a more privileged level (kernel mode) than the user (user mode); a (software) interrupt is used to make requests to the operating system. This is rarely a feature in a higher-level language, and so wrapper functions for system calls are written using inline assembler.

The following C code comprises samples including a system call wrapper in AT&T assembler syntax with the GNU Assembler. They are normally written with the aid of macros; the full code is included for clarity.

The format of basic inline assembly is very straightforward and shown below.

Example:

OR

Both asm and __asm__ are valid. __asm__ can be used if the keyword asm conflicts with something else in the program.

This example of the inline assembly is from the D programming language and computes the tangent of x using the x86's FPU instructions. This is faster than using the floating-point operations that would be emitted by the compiler, and it allows the programmer to make use of the fldpi instruction, which loads the closest approximation of pi possible on the x86 architecture.

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