The syscalls, such as sys_read(), sys_write() and sys_exit() are x64 assembly language instructions specified by the chip maker, such as Intel Corporation or American Microwave Devices Corp. assembler-dependent (e.g., NASM, YASM) operating systems calls for input/output, exit etc that need OS kernel functions. programmer defined code just like user-defined functions in C++. library functions from C/C++ that help an assembler pass control to CS kernel