An embedded assembly function definition is marked by the __asm (C and C++) or asm (C++) function qualifiers, and can be used on:

Functions declared with __asm or asm can have arguments, and return a type. They are called from C and C++ in the same way as normal C and C++ functions. The syntax of an embedded assembly function is:

__asm return-type function-name(parameter-list){    // ARM/Thumb/Thumb-2 assembler code    instruction[;instruction]    ...    instruction}

The initial state of the embedded assembler (ARM or Thumb) is determined by the initial state of the compiler, as specified on the command line. This means that:

The embedded assembler state at the start of each function is as set by the invocation of the compiler, as modified by #pragma arm and #pragma thumb pragmas.

You can change the state of the embedded assembler within a function by using explicit ARM, THUMB, or CODE16 directives in the embedded assembler function. Such a directive within an __asm function does not affect the ARM or Thumb state of subsequent __asm functions.

If you are compiling for a Thumb-2 capable processor, you can use Thumb-2 instructions when in Thumb state.

__asm int f(int i)
{
    ADD i, i, #1 // error
}

See the chapter on mixing C, C++, and assembly language in Developer Guide for more information on embedded assembly language in C and C++ sources.

#include <stdio.h>

__asm void my_strcpy(const char *src, char *dst)
{
loop
      LDRB  r2, [r0], #1
      STRB  r2, [r1], #1
      CMP   r2, #0
      BNE   loop
      BX    lr
}

int main(void)
{
    const char *a = "Hello world!";
    char b[20];
    my_strcpy (a, b);
    printf("Original string: '%s'\n", a);
    printf("Copied   string: '%s'\n", b);
    return 0;
}