On the hardware level, the processor provides us with several ways to invoke a kernel procedure to handle a system call:

• Through an interrupt (INT instruction on 32-bit systems): The operating system allocates a descriptor for an interrupt with a specific number, which points to a procedure in the kernel space that handles the interrupt in accordance with its parameters (parameters are passed via registers). One of the parameters is the index into system call table (roughly speaking, the table of pointers to specific system call handlers).
• Using the SYSENTER instruction (32-bit systems excluding WOW64 processes): Beginning with Pentium II, we are able to use the SYSENTER instruction to perform fast calls to a ring 0 procedure. This instruction is accompanied by the SYSEXIT instruction, which returns from a system call.
• Using the SYSCALL instruction (64-bit systems): This instruction was introduced by the x86_64 architecture and is only available in the long mode. This instruction allows faster transfer to a system call handler and does not access the interrupt descriptor table.