- 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.
- 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) 493 RING0_INT_FRAME # can't unwind into user space anyway 494 ASM_CLAC 495 pushl_cfi %eax # save orig_eax 496 SAVE_ALL 497 GET_THREAD_INFO(%ebp) 498 # system call tracing in operation / emulation 499 testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%ebp) 500 jnz syscall_trace_entry 501 cmpl $(NR_syscalls), %eax 502 jae syscall_badsys 503 syscall_call: 504 call *sys_call_table(,%eax,4)