When a User Mode process invokes a system call, the CPU switches to Kernel Mode and starts the execution of a kernel function. As we will see in the next section, in the 80 x 86 architecture a Linux system call can be invoked in two different ways. The net result of both methods, however, is a jump to an assembly language function called the system call handler.
Because the kernel implements many different system calls, the User Mode process must pass a parameter called the system call number to identify the required system call; the eax register is used by Linux for this purpose. As we'll see in the section "Parameter Passing" later in this chapter, additional parameters are usually passed when invoking a system call.