In addition to entering through system calls, the system can enter kernel mode for other reasons, such as in response to a device interrupt, or to take care of a situation that could not be handled in user mode. A transfer of control to the kernel is achieved in one of three ways:
Through a system call
As the result of an interrupt
As the result of a processor trap
We defined a system call as the mechanism by which a user process requests a kernel service, for example, to read from a file. System calls are typically initiated from user mode by either a trap instruction or a software interrupt, depending on the microprocessor and platform. On SPARC-based platforms, system calls are initiated by issuing a specific trap instruction ...