Because this is actually an interrupt however the

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Unformatted text preview: of Exceptions The processor receives exceptions from three sources: • • • Processor-detected program-error exceptions. Software-generated exceptions. Machine-check exceptions. PROGRAM-ERROR EXCEPTIONS The processor generates one or more exceptions when it detects program errors during the execution in an application program or the operating system or executive. The Intel Architecture defines a vector number for each processor-detectable exception. The exceptions are further classified as faults, traps, and aborts (refer to Section 5.3., “Exception Classifications”). SOFTWARE-GENERATED EXCEPTIONS The INTO, INT 3, and BOUND instructions permit exceptions to be generated in software. These instructions allow checks for specific exception conditions to be performed at specific points in the instruction stream. For example, the INT 3 instruction causes a breakpoint exception to be generated. The INT n instruction can be used to emulate a specific exception in software, with one limitation. If the n operand in the INT n instruction contains a vector for one of the Intel Architecture exceptions, the processor will generate an interrupt to that vector, which will in turn invoke the exception handler associated with that vector. Because this is actually an interrupt, however, the processor does not push an error code onto the stack, even if a hardware-generated exception for that vector normally produces one. For those exceptions that produce an error code, the exception handler will attempt to pop an error code from the stack while handling the exception. If the INT n instruction was used to emulate the generation of an exception, the handler will pop off and discard the EIP (in place of the missing error code), sending the return to the wrong location. 5-3 INTERRUPT AND EXCEPTION HANDLING MACHINE-CHECK EXCEPTIONS The P6 family and Pentium® processors provide both internal and external machine-check mechanisms for checking the operation of the internal chip hardware and bus transactions. These mechanisms constitute extended (implementation dependent) exception mechanisms. When a machine-check error is detected, the processor signals a machine-check exception (vector 18) and returns an error code. Refer to “Interrupt 18—Machine Check Exception (#MC)” at the end of this chapter and Chapter 13, Machine-Check Architecture, for a detailed description of the machine-check mechanism. 5.2. EXCEPTION AND INTERRUPT VECTORS The processor associates an identification number, called a vector, with each exception and interrupt. Table 5-1 shows the assignment of exception and interrupt vectors. This table also gives the exception type for each vector, indicates whether an error code is saved on the stack for an exception, and gives the source of the exception or interrupt. The vectors in the range 0 through 31 are assigned to the exceptions and the NMI interrupt. Not all of these vectors are currently used by the processor. Unassigned vectors in this range are reserved for possible future uses. Do not use the reserved vectors. The vectors in the range 32 to 255 are designated as user-defined interrupts. These interrupts are not reserved by the Intel Architecture and are generally assigned to external I/O devices and to permit them to signal the processor through one of the external hardware interrupt mechanisms described in Section 5.1.1., “Sources of Interrupts” 5.3. EXCEPTION CLASSIFICATIONS Exceptions are classified as faults, traps, or aborts depending on the way they are reported and whether the instruction that caused the exception can be restarted with no loss of program or task continuity. Faults A fault is an exception that can generally be corrected and that, once corrected, allows the program to be restarted with no loss of continuity. When a fault is reported, the processor restores the machine state to the state prior to the beginning of execution of the faulting instruction. The return address (saved contents of the CS and EIP registers) for the fault handler points to the faulting instruction, rather than the instruction following the faulting instruction. Note: There are a small subset of exceptions that are normally reported as faults, but under architectural corner cases, they are not restartable and some processor context will be lost. An example of these cases is the execution of the POPAD instruction where the stack frame crosses over the the end of the stack segment. The exception handler will see that the CS:EIP has been restored as if the POPAD instruction had not executed however internal processor state (general purpose registers) will have been modified. These corner cases are 5-4 INTERRUPT AND EXCEPTION HANDLING considered programming errors and an application causeing this class of exceptions will likely be terminated by the operating system. Traps A trap is an exception that is reported immediately following the execution of the trapping instruction. Traps allow execution of a program...
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This note was uploaded on 06/07/2013 for the course ECE 1234 taught by Professor Kwhon during the Spring '10 term at Berkeley.

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