How a system call is executed in X86 architecture?

• A vector in the interrupt descriptor table (IDT) is used to invoke the system call. 
• Only one vector is allocated for the system calls.
• Immediately a question comes into our mind that since there is only one vector so how so many system calls can be serviced?
• This is done by having a generic function (we can say an ISR) which will multiplex all other system calls. 
• That means when an interrupt (software interrupt using INT instruction) is raised on this vector, the generic function will be called and a system call number is passed as an argument to this function.
• This generic function uses this system call number as an index into the sys_call_table array and gets the address (function pointer) of the system call and invokes that system call. 
• arch/x86/kernel/syscall_64.c#L25
• Interrupt vector used for the system call is 0x80(128), i.e interrupt descriptor table's 128th entry.(80 in hex is 128 in decimal).
•  128th entry in the IDT table contains the address of the system_call() function. system_call() is defined in arch/x86/kernel/entry_32.S.

 ENTRY(system_call)
         RING0_INT_FRAME                 # can't unwind into user space anyway
         ASM_CLAC
         pushl_cfi %eax                  # save orig_eax
         SAVE_ALL
         GET_THREAD_INFO(%ebp)
                                         # system call tracing in operation / emulation
         testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%ebp)
         jnz syscall_trace_entry
         cmpl $(NR_syscalls), %eax
         jae syscall_badsys
 syscall_call:
         call *sys_call_table(,%eax,4)

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