X86 Accumulation GAS Syntax

 13 October 02:21   

    Examples in this commodity are created using the AT&T accumulation syntax acclimated in GNU AS. The capital advantage of using this syntax is its affinity with the GCC inline accumulation syntax. However, this is not the alone syntax that is acclimated to represent x86 operations. For example, NASM uses a altered syntax to represent accumulation mnemonics, operands and acclamation modes, as do some s. The AT&T syntax is the accepted on Unix-like systems but some assemblers use the Intel syntax, or can acquire both.

    GAS instructions about accept the anatomy mnemonic source, destination. For instance, the afterward mov instruction:

     movb $0x05, %al

    will move the amount 5 into the annals al.

    GAS accumulation instructions are about suffixed with the belletrist b, s, w, l, q or t to actuate what admeasurement operand is getting manipulated.

    If the suffix is not specified, and there are no anamnesis operands for the instruction, GAS infers the operand admeasurement from the admeasurement of the destination annals operand (the final operand).

    When referencing a register, the annals needs to be prefixed with a %. Connected numbers charge to be prefixed with a $.

    This area is accounting as a abbreviate addition to GNU as (gas), an assembler that can accumulate the x86 accumulation language. gas is allotment of the [http://www.gnu.org/ GNU Project], which gives it the afterward nice properties:

    If you are using a computer with the Linux operating system, affairs are you already accept gas installed on your system. If you are using a computer with the Windows operating system, you can install gas and additional advantageous programming utilities by installing [http://www.cygwin.com/ Cygwin] or [http://www.mingw.org/ Mingw]. The butt of this addition assumes you accept installed gas and understand how to accessible a command-line interface and adapt files.

    Since accumulation accent corresponds anon to the operations a CPU performs, a anxiously accounting accumulation accepted may be able to run abundant faster than the aforementioned accepted accounting in a higher-level language, such as C. On the additional hand, accumulation routines about yield added accomplishment to address than the agnate accepted in C. Thus, a archetypal adjustment for bound autograph a program that performs able-bodied is to first address the program in a high-level accent (which is easier to address and debug), then carbon called routines in accumulation accent (which performs better). A acceptable first move to afterlight a C accepted in accumulation accent is to use the C compiler to automatically accomplish the accumulation language. Not alone does this accord you an accumulation book that compiles correctly, but it aswell ensures that the accumulation accepted does absolutely what you advised it to.

    We will now use the GNU C compiler to accomplish accumulation code, for the purposes of analytical the gas accumulation accent syntax.

    Here is the archetypal Hello, apple program, accounting in C:

     #include

     int main(void)

    Save that in a book alleged hello.c, then blazon at the prompt:

     gcc -o hello_c.exe hello.c

    This should abridge the C book and make an executable book alleged hello_c.exe. If you get an error, create abiding that the capacity of hello.c are correct.

    Now you should be able to blazon at the prompt:

     ./hello_c.exe

    and the program should book Hello, world! to the console.

    Now that we understand that hello.c is typed in accurately and does what we want, lets accomplish the agnate x86 accumulation language. Blazon the afterward at the prompt:

     gcc -S hello.c

    This should make a book alleged hello.s (.s is the book addendum that the GNU arrangement gives to accumulation files). To abridge the accumulation book into an executable, type:

     gcc -o hello_asm.exe hello.s

    (Note that gcc calls the assembler (as) and the linker (ld) for us.) Now, if you blazon the afterward at the prompt:

     ./hello_asm.exe

    this program should aswell book Hello, world! to the console. Not surprisingly, it does the aforementioned affair as the aggregate C file.

    Lets yield a attending at what is central hello.s:

     .file hello.c

     .def ___main; .scl 2; .type 32; .endef

     .text

     LC0:

     .ascii Hello, world!12