System Calls in OS/161

I'm been walking through the code in OS/161 with respect to how systems calls are executed. From what I can see, a system call (e.g. reboot()) is actually translated by the OS/161 kernel into a call to sys_reboot(). Similarly, a call to fork() would be translated to a call to sys_fork().

Is my understanding correct?

Thanks.

Each system call has a unique identifying number, in OS161 these system call numbers are defined in kern/include/kern/syscall.h:

#define SYS_reboot       119  

The library procedure reboot() places the syscall number in a register (v0) and issues a trap to the OS, the syscall handler receives from the assembly-language exception handler a data structure called trapframe which contains, among other information, the system call number.

This number is used in a switch case statement to select the function:

void syscall(struct trapframe *tf)
    ...
    callno = tf->tf_v0;
    ...
    switch (callno) {
        case SYS_reboot:
        err = sys_reboot(tf->tf_a0);
        break;