$($a$)$ How does the OS kernel distinguish the system calls? $($for instance, how does the kernel know that the user program is trying to open a file in of trying to establish a network connection?$)$

$($b$)$ How do we pass the system call parameters from a user$-$level program to the kernel?

$($c$)$ On a single$-$core processor, can the system calls from different processes execute concurrently? Why or why not?

$($d$)$ On a multi$-$core processor, can the system calls from different processes execute in parallel? Why or why not?