Which assembler to use

System Calls in bit Linux There are some systems with bit builds of Linux out there still. Although it might be interesting to show some examples for historical reasons, this introduction is probably better kept short. Sometimes you might like to use your favorite C library functions in your assembly code.

This should be trivial because the C library functions are all stored in a C library, such as libc. Technically the code is probably in a dynamic library, like libc. Still, all we have to do is place calls to C functions in our assembly language program, and link with the static C library and we are set. So if we link with a C library, all we have to do is define main and end with a ret instruction!

Here is a simple example in NASM, which illustrates calling puts. To assemble and run: ; ; nasm -fmacho64 hola. C library functions have underscores, and we had to say default rel for some strange reason, which you can read about in the NASM documentation.

We will follow the plan of the previous section and first look at programs that just use system calls and then programs that use a C library. Win32 defines thousands of functions! The code for these functions is spread out in many different dynamic libraries, but the majority of them are in KERNEL DLL which exist on all Windows installations. The interrupt to execute system calls on the x86 processor is hex 2E, with EAX containing the system call number and EDX pointing to the parameter table in memory.

However, according to z0mbie , the actually system call numbers are not consistent across different operating systems, so, to write portable code you should stick to the API calls in the various system DLLs. The comments instruct us to assemble into an object format of "win32" not "coff"! Of course you can use any linker you want, but ld comes with gcc and you can download a whole Win32 port of gcc for free. We pass the starting address to the linker, and specify the static library libkernel This static library is part of the Win32 gcc distribution, and it contains the right calls into the system DLLs.

In fact the differences between the two programs are really only syntactic. To assemble this code, do. The -c option is important! It tells gcc to assemble but not link.

Without the -c option, gcc will try to link the object file with a C runtime library. Since we are not using a C runtime library, and in fact are specifying our own starting point, and cleaning up ourselves with ExitProcess we definitely want to link ourselves. The linking step is the same as the NASM example; the only difference is that gcc produces win32 object files with extension.

Anything less than version 6. Here is the version of the hello program in MASM. The processor. As before we have to specify an entry point and pass it to the linker.

Assemble with. After you buy those you link your code with. To get this to work, kernel GAS can produce code for many different platforms. It has been written in C but only generates code for x86 and its predecessors. The Flat Assembler, or FASM , is a self assembling assembler written entirely in assembly language and uses Intel-syntax with its own macro system. It only produces code for ia32 and amd64 platforms, though a fork for ARM systems exists.

GAS is included in the GNU binutils and therefore does not introduce new dependencies to your project. If you want to write an project in assembly language entirely, the latter two are usually the tools of choice. However, FASM being self-assembling and only needs a C standard library to work gives it a slight advantage here as it is way easier to port.

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