Inline Assembly
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Inline assembly is a way of having complete control how code is generated, and is very useful when optimizing functions or accessing hardware directly.


Assembly Template

Insert inline assembly into your C/C++ functions by using a basic assembly template that look like this:

00 asm("...;"
01     "...;"
02     "...;"
03     "...;"
04     : "=r"(...), "=r"(...), ...
05     : "r"(...), "r"(...)
06     : "...");

Lines 0 to 3 are the assembly code, the logic of your function. Each line of assembly code ends in ;.

Line 4 contains the output variables and line 5 are the input variables. In quotes you can specify constraint for the variables. = indicate that the variables will be written to. r tells compiler that we want to place the value to a register. In ( ) we insert the variable names that will be mapped.

Variables are mapped to %? variables in the template in the order listed and ? is replaced by a number between 0 and 9. If input and output refer to the same variable, then instead of specifying r for constraint, you will list the number that the output variable is mapped to.

Line 6 indicate registers that will be overwritten, so that compiler can take steps to preserve values before entering inline assembly code.


To convert the following C code into inline assembly:

z1 = ((z2 + z3) * 4433);
tmp2 = z1 + (z3 * -15137);
tmp3 = z1 + (z2 * 6270);

Here is one translation:

asm("addu %1, %4, %5;
     li %0, 4433;
     multu %1, %0;
     mflo  %1;

     li    %0, -15137;
     mult  %5, %0;
     mflo  %0;
     add   %2, %1 %0;

     li    %0, 6270;
     multu %4, %0;
     mflo  %0;
     addu  %3, %1, %0;
    :"=r"(zz), "=r"(z1), "=r"(tmp2), "=r"(tmp3)
    :"r"(z2), "r"(z3), "0"(zz)

Variable zz is a temporary variable where we load constants to. The last variable on input line shows how to map input to output (in this case, by declaring "0" to map to %0).

Notes and Restrictions

  • Only 10 registers can be mapped.
  • If temporary variable is required, you need to map it by listing it in the output variables line.
  • It seems that long blocks of inline assembly code runs slower than smaller blocks of the same code. You should experiment and confirm with the compiler you are using.
  • Use debugger or compiler flags to generate disassembly so that you can check the results.
  • If you are getting compiler warnings about AT registers being used, it's probably because you typed $1 instead of %1 in your assembly code, and it is clobbering up a reserved MIPS register used by assembler.
  • You can specify assembler directives to enforce certain behavior. For example, .set noreorder to make sure code is executed exactly in the order you specify.
  • If you want to use instructions that is not supported by the compiler, you will need to create a macro (use .macro and .endm directives), and within the macro use .word to map out the opcode. This may not work with inline assembly, so check your results.
  • Atomic operations can be achieved via special assembly instructions. For ARM, use swp or ldrex/strex; for MIPS, use ll/sc.
  • To generate code listing of mixed C/assembly, compile with -g -Wa,-a,-ad flags.
  • If the application breaks between recompile, or if inline functions do not work, then it is possible that the constraint modifiers need to be checked carefully.


  1. Dean Elsner, Jay Fenlason & friends. Using Assembly Language in Linux. May 19, 2008 <>.
  2. Sandeep.S. GCC-Inline-Assembly-HOWTO. May 15, 2008 <>.
  3. Keith Wesolowski. GCC inline assembly, part 2. February 26, 2009 <>.
  4. Gary Shute. MIPS Instruction Coding. March 11, 2009 <>.

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