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 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. That is:

You can change the state of the embedded assembler by using explicit ARM, THUMB, or CODE16 directives in the embedded assembler function. The directives have effect until the next such directive is encountered. That is, a directive will set ARM or Thumb state for all subsequent __asm functions until it is over-ridden.

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

#include <stdio.h>
__asm void my_strcpy(const char *src, const char *dst) {
loop
      LDRB  r3, [r0], #1
      STRB  r3, [r1], #1
      CMP   r3, #0
      BNE   loop
      MOV   pc, lr
}
void main()
{
    const char *a = "Hello world!";
    char b[20];
    my_strcpy (a, b);
    printf("Original string: '%s'\n", a);
    printf("Copied   string: '%s'\n", b);
}