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Below is a slightly simplified version of the Linux code that is called to handle a system call trap.
The first part of the code (starting at system_call) saves the registers of the user process and plays around with the memory management registers so that the kernel's internal data is accessible. It also finds the process table entry for this user process.
The trap instruction that caused the entry to the kernel has a parameter that specifies which system call is being invoked. The code starting at do_call checks to see if this number is in range, and then calls the function associated with this system call number. When this function returns, the return value (stored in the eax register) is saved in the place where all the other user registers are stored. As a result, when control is transferred from the kernel back to the user process, the return value will be in the right place.
After the system call is complete, it is time to return to the user process. There are two choices at this point: (1) either return directly the the user process that made the system call or (2) go through the dispatcher to select the next process to run. ret_from_sys_call
system_call:
##----Save orig_eax: system call number
# used to distinguish process that entered# kernel via syscall from one that entered
# via some other interrupt#
pushl %eax#
#----Save the user's registers#
pushl %espushl %ds
pushl %eaxpushl %ebp
pushl %edipushl %esi
pushl %edxpushl %ecx
pushl %ebx#
#----Set up the memory segment registers so that the kernel's# data segment can be accessed.
#movl $(__KERNEL_DS),%edx
movl %edx,%dsmovl %edx,%es
##----Load pointer to task structure in EBX. The task structure
# resides below the 8KB per-process kernel stack.#
movl $-8192, %ebxandl %esp, %ebx
##----Check to see if system call number is a valid one, then
# look-up the address of the kernel function that handles this# system call.
#do_call:
cmpl $(NR_syscalls),%eaxjae badsys
call *SYMBOL_NAME(sys_call_table)(,%eax,4)# Put return value in EAX of saved user context
movl %eax,EAX(%esp)#
#----If we can return directly to the user, then do so, else go to# the dispatcher to select another process to run.
#ret_from_sys_call:
cli # Block interrupts; iret effectively re-enables themcmpl $0,need_resched(%ebx)
jne reschedule# restore user context (including data segments)
popl %ebxpopl %ecx
popl %edxpopl %esi
popl %edipopl %ebp
popl %eaxpopl %ds
popl %esaddl $4,%esp # ignore orig_eax
iretreschedule:
call SYMBOL_NAME(schedule)jmp ret_from_sys_call
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