inline gcc assembly

http://wiki.osdev.org/Inline_Assembly/Examples

http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html

E.G.

---

int a=10, b;
        asm ("movl %1, %%eax; 
              movl %%eax, %0;"
             :"=r"(b)        /* output */
             :"r"(a)         /* input */
             :"%eax"         /* clobbered register */
             );       

---

Above code will have compile errors, but if you put assembly code line2 and line3 together, it will compile. Or you could change it to:

---

int a=10, b;
        asm ("movl %1, %%eax;" 

             "movl %%eax, %0;"
             :"=r"(b)        /* output */
             :"r"(a)         /* input */
             :"%eax"         /* clobbered register */
             );       

This time, it will work.


Explaining:

Here what we did is we made the value of ’b’ equal to that of ’a’ using assembly instructions. Some points of interest are:

• "b" is the output operand, referred to by %0 and "a" is the input operand, referred to by %1.
• "r" is a constraint on the operands. We’ll see constraints in detail later. For the time being, "r" says to GCC to use any register for storing the operands. output operand constraint should have a constraint modifier "=". And this modifier says that it is the output operand and is write-only.
• There are two %’s prefixed to the register name. This helps GCC to distinguish between the operands and registers. operands have a single % as prefix.
• The clobbered register %eax after the third colon tells GCC that the value of %eax is to be modified inside "asm", so GCC won’t use this register to store any other value.

When the execution of "asm" is complete, "b" will reflect the updated value, as it is specified as an output operand. In other words, the change made to "b" inside "asm" is supposed to be reflected outside the "asm".

Now we may look each field in detail.