When a user program makes a system call, it can’t execute privileged instructions directly, so it triggers a software interrupt (or trap).

Here’s roughly what happens:

1. The CPU switches from user mode to kernel mode and jumps to a fixed location in memory (the interrupt vector) where the ISR for system calls lives.

2. The ISR (Interrupt Service Routine) runs some setup: it saves registers, switches to the kernel stack, and checks which system call was requested.

3. The ISR then uses the system call number to look up the system call table, which is basically an array of pointers to all system call handler functions in the kernel.

4. The kernel executes the actual system call handler, performs the operation, and stores the return value.

5. Finally, the CPU restores the user program’s state and goes back to user mode, returning control to the program.

So, the ISR isn’t the system call itself—it’s just the bridge from the trap to the kernel function. The system call table is where the kernel finds the correct function to run.