Linux 64-bit accepts both the old IA-32 int 0x80 system calls and also the new syscall instruction, which should be used instead as it is faster. Support for the old calls likely exists so that you can run IA-32 elf files directly.
syscall system calls have different numbers than the old IA-32, so watch out.
Syscall parameters are passed as follows:
rax: system call numberrdi: parameter 1rsi: parameter 2rdxr10r8r9
The return value is stored in rax.