4.5. Exception Handling

Most exceptions issued by the CPU are interpreted by Linux as error conditions. When one of them occurs, the kernel sends a signal to the process that caused the exception to notify it of an anomalous condition. If, for instance, a process performs a division by zero, the CPU raises a "Divide error " exception, and the corresponding exception handler sends a SIGFPE signal to the current process, which then takes the necessary steps to recover or (if no signal handler is set for that signal) abort.

There are a couple of cases, however, where Linux exploits CPU exceptions to manage hardware resources more efficiently. A first case is already described in the section "Saving and Loading the FPU, MMX, and XMM Registers" in Chapter 3. The "Device not available " exception is used together with the TS flag of the cr0 register to force the kernel to load the floating point registers of the CPU with new values. A second case involves the "Page Fault " exception, which is used to defer allocating new page frames to the process until the last possible moment. The corresponding handler is complex because the exception may, or may not, denote an error condition (see the section "Page Fault Exception Handler" in Chapter 9).

Exception handlers have a standard structure consisting of three steps:

• Save the contents of most registers in the Kernel Mode stack (this part is coded in assembly language).
• Handle the exception by means of a high-level C function. 2.
• Exit from the handler by means of the ret_from_exception( ) function.