#First Program "Hello World" in Linux ASM Programming.
Syscalls:-
This section will outline the use of linux syscalls in assembly language. Syscalls consist of all the functions in the second section of the manual pages located in /usr/man/man2. They are also listed in: /usr/include/sys/syscall.h. A great list is at http://www.linuxassembly.org/syscall.html. These functions can be executed via the linux interrupt service: int $0x80.
##Syscalls with < 6 args.
For all syscalls, the syscall number goes in %eax. For syscalls that have less than six args, the args go in %ebx,%ecx,%edx,%esi,%edi in order. The return value of the syscall is stored in %eax.
The syscall number can be found in /usr/include/sys/syscall.h. The macros are defined as SYS_ i.e. SYS_exit, SYS_close, etc.
##Syscalls with > 5 args.
Syscalls whos number of args is greater than five still expect the syscall number to be in %eax, but the args are arranged in memory and the pointer to the first arg is stored in %ebx.
If you are using the stack, args must be pushed onto it backwards, i.e. from the last arg to the first arg. Then the stack pointer should be copied to %ebx. Otherwise copy args to an allocated area of memory and store the address of the first arg in %ebx.
For better Understanding - http://asm.sourceforge.net/articles/linasm.html
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Compilation:
as -gstabs -o filename.o filename.s
Linking:
ld -o filename filename.o
Execution:
./filename
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NOTE: ALL EXAMPLES FROM HERE WILL BE IN AT&T SYNTAX