Unix Signal - Relationship With Hardware Exceptions

Relationship With Hardware Exceptions

A process's execution may result in the generation of a hardware exception, for instance, if the process attempts to divide by zero or incurs a TLB miss. In Unix-like operating systems, this event automatically changes the processor context to start executing a kernel exception handler. In case of some exceptions, such as a page fault, the kernel has sufficient information to fully handle the event itself and resume the process's execution. Other exceptions, however, the kernel cannot process intelligently and it must instead defer the exception handling operation to the faulting process. This deferral is achieved via the signal mechanism, wherein the kernel sends to the process a signal corresponding to the current exception. For example, if a process attempted integer divide by zero on an x86 CPU, a divide error exception would be generated and cause the kernel to send the SIGFPE signal to the process. Similarly, if the process attempted to access a memory address outside of its virtual address space, the kernel would notify the process of this violation via a SIGSEGV signal. The exact mapping between signal names and exceptions is obviously dependent upon the CPU, since exception types differ between architectures.