分类： LINUX
2011-04-12 16:51:13
Copyright (C)2003 Sandeep S.
This document is free; you can redistribute and/or modify this under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This document is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
Kindly forward feedback and criticism to . I will be indebted to anybody who points out errors and inaccuracies in this document; I shall rectify them as soon as I am informed.
I express my sincere appreciation to GNU people for providing such a great feature. Thanks to Mr.Pramode C E for all the helps he did. Thanks to friends at the Govt Engineering College, Trichur for their moral-support and cooperation, especially to Nisha Kurur and Sakeeb S. Thanks to my dear teachers at Govt Engineering College, Trichur for their cooperation.
Additionally, thanks to Phillip, Brennan Underwood and colin@nyx.net; Many things here are shamelessly stolen from their works.
We are here to learn about GCC inline assembly. What this inline stands for?
We can instruct the compiler to insert the code of a function into the code of its callers, to the point where actually the call is to be made. Such functions are inline functions. Sounds similar to a Macro? Indeed there are similarities.
What is the benefit of inline functions?
This method of inlining reduces the function-call overhead. And if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function’s code needs to be included. The effect on code size is less predictable, it depends on the particular case. To declare an inline function, we’ve to use the keyword inline in its declaration.
Now we are in a position to guess what is inline assembly. Its just some assembly routines written as inline functions. They are handy, speedy and very much useful in system programming. Our main focus is to study the basic format and usage of (GCC) inline assembly functions. To declare inline assembly functions, we use the keyword asm.
Inline assembly is important primarily because of its ability to operate and make its output visible on C variables. Because of this capability, "asm" works as an interface between the assembly instructions and the "C" program that contains it.
GCC, the GNU C Compiler for Linux, uses AT&T/UNIX assembly syntax. Here we’ll be using AT&T syntax for assembly coding. Don’t worry if you are not familiar with AT&T syntax, I will teach you. This is quite different from Intel syntax. I shall give the major differences.
The direction of the operands in AT&T syntax is opposite to that of Intel. In Intel syntax the first operand is the destination, and the second operand is the source whereas in AT&T syntax the first operand is the source and the second operand is the destination. ie,
"Op-code dst src" in Intel syntax changes to
"Op-code src dst" in AT&T syntax.
Register names are prefixed by % ie, if eax is to be used, write %eax.
AT&T immediate operands are preceded by ’$’. For static "C" variables also prefix a ’$’. In Intel syntax, for hexadecimal constants an ’h’ is suffixed, instead of that, here we prefix ’0x’ to the constant. So, for hexadecimals, we first see a ’$’, then ’0x’ and finally the constants.
In AT&T syntax the size of memory operands is determined from the last character of the op-code name. Op-code suffixes of ’b’, ’w’, and ’l’ specify byte(8-bit), word(16-bit), and long(32-bit) memory references. Intel syntax accomplishes this by prefixing memory operands (not the op-codes) with ’byte ptr’, ’word ptr’, and ’dword ptr’.
Thus, Intel "mov al, byte ptr foo" is "movb foo, %al" in AT&T syntax.
In Intel syntax the base register is enclosed in ’[’ and ’]’ where as in AT&T they change to ’(’ and ’)’. Additionally, in Intel syntax an indirect memory reference is like
section:[base + index*scale + disp], which changes to
section:disp(base, index, scale) in AT&T.
One point to bear in mind is that, when a constant is used for disp/scale, ’$’ shouldn’t be prefixed.
Now we saw some of the major differences between Intel syntax and AT&T syntax. I’ve wrote only a few of them. For a complete information, refer to GNU Assembler documentations. Now we’ll look at some examples for better understanding.
+------------------------------+------------------------------------+
| Intel Code