asm declaration

asm-declaration gives the ability to embed assembly language source code within a C++ program. This declaration is conditionally-supported and implementation defined, meaning that it may not be present and, even when provided by the implementation, it does not have a fixed meaning.

Syntax

asm ( string_literal ) ;

Explanation

The string_literal is typically a short program written in assembly language, which is executed whenever this declaration is executed. Different C++ compilers have wildly varying rules for asm-declarations, and different conventions for the interaction with the surrounding C++ code.

As other block declarations, this declaration can appear inside a block (a function body or another compound statement), and, as all other declarations, this declaration can also appear outside a block.

Examples

Demonstrates two kinds of inline assembly syntax offered by the GCC compiler. This program will only work correctly on x86_64 platform under Linux.

#include <iostream>
 
extern "C" int func();
// the definition of func is written in assembly language
asm(".globl func\n\t"
    ".type func, @function\n\t"
    "func:\n\t"
    ".cfi_startproc\n\t"
    "movl $7, %eax\n\t"
    "ret\n\t"
    ".cfi_endproc");
 
int main()
{
    int n = func();
    // extended inline assembly
    asm ("leal (%0,%0,4),%0"
         : "=r" (n)
         : "0" (n));
    std::cout << "7*5 = " << n << std::endl; // flush is intentional
 
    // standard inline assembly
    asm ("movq $60, %rax\n\t" // the exit syscall number on Linux
         "movq $2,  %rdi\n\t" // this program returns 2
         "syscall");
}

Output:

7*5 = 35

External links

Links:

http://en.cppreference.com/w/cpp/language/asm

doc_CPP

2016-10-11 09:57:33

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