Regardless of the assembler, object file format, linker or operating system you use, the programming process is always the same:.
Each assembly language file is assembled into an "object file" and the object files are linked with other object files to form an executable. A "static library" is really nothing more than a collection of probably related object files. This document does not cover how to use all the different assemblers; you need to read the documentation that comes with them. We will, however, give step-by-step instructions and complete examples of all three of these assemblers for a few extremely simple programs.
Some linkers out there include. In our first example we will use system calls for writing to a file call number 1 and exiting a process call number Here it is in the NASM assembly language:. If you just enter " gcc hello. You can suppress the link step with the -c option to gcc , or do the assembly and linking in one step by telling the linker not to use the C library with -nostdlib.
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. Although there's not much to it, you'll find an icon and all the usual Windows " Properties " from the right-click menu in Explorer , and here's the info that many programming utilities will find:.
He has many new web pages at his site dealing with Assembly under Linux if you'd like to learn programming for that OS! This is not for beginners as Hutch says, but there is a forum for intermediate users to get help and discuss what they're working on.
RadASM [ New link provided by a reader. This page also contains links to other important Win32 resources!
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