Why can User call a system call directly?

Question

Before I ask the question, the following is what I know.

1. The system call is in the kernel area.
2. The kernel area cannot be used (accessed) directly by the user.

There are two ways to call a system call.

1. direct call
2. wrapping function (API) that contains system call (2. process: (User Space) wrapping function -> system call interface -> (Kernel Space) System call)

So, in 1. case) How can User use the kernel area directly? Or I wonder if there's anything I'm mistaken about.

1. open sns question
2. internet search
3. read operating system concepts 10th (page. 64)

Answer 1

The default is that nothing in user-space is able to execute anything in kernel space. How that works depends on the CPU and the OS, but likely involves some kind of "privilege level" that must be matched or exceeded before the CPU will allow software to access the kernel's part of virtual memory.

This default behavior alone would be horribly useless. For an OS to work there must be some way for user-space to transfer control/execution to (at least one) clearly marked and explicitly allowed kernel entry point. This also depends on the OS and CPU.

For example; for "all 80x86" (including all CPUs and CPU modes) an OS can choose between:

• a software interrupt (interrupt gate or trap gate)
• an exception (e.g. breakpoint exception)
• a call gate
• a task gate
• the sysenter instruction
• the syscall instruction

..and most modern operating system choose to use the syscall instruction now.

All of these possibilities share 2 things in common:

a) There is an implied privilege level switch done by the CPU as part of the control transfer

b) The caller is unable to specify the address they're calling. Instead it's set by the kernel (e.g. during the kernel's initialization).