Inside a System Call: The Execution Path

The previous chapter described a system call as a controlled request from an application to the kernel.

Now we will follow that request across the protection boundary.

Consider this C statement:

It looks like an ordinary function call. Underneath, the program prepares a kernel request, enters privileged execution, switches to a protected kernel stack, runs kernel code, and eventually returns to the instruction after read.

The complete path is:

The exact instructions and register names depend on the processor architecture. This chapter uses Linux on x86-64 as its concrete example while keeping the overall model applicable to other systems.