void syscall(void) { int num; struct proc *curproc = myproc(); num = curproc->tf->eax; if(num > && num tf->eax = syscalls[num](); } else { cprintf("%d %s: unknown sys call %d ", curproc->pid, curproc->name, num); curproc->tf->eax = -1; } How is the system call number (read into num) first loaded into curproc->tf-Seax? the kernel copies it there as a result of a preceding call to argint the kernel saves all the general purpose registers onto the kernel stack as part of the trap frame, after an interrupt O it is saved onto the kernel stack by the hardware as part of the int instruction behavior the user places the argument to the system call here before calling int