2.18 Interface to assembly language ¶

The interface for GNU Modula-2 to assembly language is almost identical to GNU C. The only alterations are that the keywords asm and volatile are in capitals, following the Modula-2 convention.

A simple, but highly non optimal, example is given below. Here we want to add the two CARDINALs foo and bar together and return the result. The target processor is assumed to be executing the x86_64 instruction set.

PROCEDURE Example (foo, bar: CARDINAL) : CARDINAL ;
VAR
   myout: CARDINAL ;
BEGIN
   ASM VOLATILE ("movl %1,%%eax; addl %2,%%eax; movl %%eax,%0"
      : "=rm" (myout)            (* outputs *)
      : "rm" (foo), "rm" (bar)   (* inputs  *)
      : "eax") ;                 (* we trash *)
   RETURN( myout )
END Example ;

For a full description of this interface we refer the reader to the GNU C manual.

See Extensions to the C Language Family.

The same example can be written using the newer extensions of naming the operands rather than using numbered arguments.

PROCEDURE Example (foo, bar: CARDINAL) : CARDINAL ;
VAR
   myout: CARDINAL ;
BEGIN
   ASM VOLATILE (
    "movl %[left],%%eax; addl %[right],%%eax; movl %%eax,%[output]"
      : [output] "=rm" (myout)                  (* outputs *)
      : [left] "rm" (foo), [right] "rm" (bar)   (* inputs  *)
      : "eax") ;                                (* we trash *)
   RETURN( myout )
END Example ;

Both examples generate exactly the same code. It is worth noting that the specifier “rm” indicates that the operand can be either a register or memory. Of course you must choose an instruction which can take either, but this allows the compiler to make more efficient choices depending upon the optimization level.