6.2.1. Embedded assembler syntax

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.


Argument names are permitted in the parameter list, but they cannot be used in the body of the embedded assembly function. For example, the following function uses integer i in the body of the function, but this is not valid in assembly:

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

You can use, for example, r0 instead of i.

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.

Embedded assembler example

Example 6.1 shows a string copy routine as an embedded assembler routine.

Example 6.1. String copy with embedded assembler

#include <stdio.h>

__asm void my_strcpy(const char *src, char *dst)
      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;

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