Updated: 11 December 1998

VAX MACRO and Instruction Set Reference Manual

The following are possible serious system failures:

• Kernel stack not valid abort
• Interrupt stack not valid halt
• Machine check exception

These system failures are described in the following sections.

E.6.1 Kernel Stack Not Valid Abort

The kernel stack not valid abort is an exception indicating that the kernel stack was not valid while the processor was pushing information onto it during the initiation of an exception or interrupt. This is usually an indication of a stack overflow or other operating system error. During this process, the attempted exception is transformed into an abort that uses the interrupt stack. Only the PSL and PC of the original exception are pushed onto the interrupt stack. The interrupt priority level (IPL) is raised to 1F (hex). If the exception vector bits <1:0> are not both 1, the operation of the processor is UNDEFINED.

Software can abort the process without aborting the system. However, because of the lost information, the process cannot be continued. If the kernel stack is not valid during the normal execution of an instruction (including CHMx or REI), the normal memory management fault is initiated.

E.6.2 Interrupt Stack Not Valid Halt

An interrupt stack not valid halt results when the interrupt stack was not valid or a memory error occurred while the processor was pushing information onto the interrupt stack during the initiation of an exception or interrupt. No further interrupt requests are acknowledged on the processor. The processor leaves the PC, the PSL, and the reason for the halt in registers so that they are available to a debugger, to the normal bootstrap routine, or to an optional watchdog bootstrap routine. A watchdog bootstrap routine can cause the processor to leave the halted state.

E.6.3 Machine Check Exception

A machine check exception indicates that the processor detected an internal error. As is usual for exceptions, a machine check is taken regardless of current interrupt priority level (IPL). The machine check exception vector (bits 0 to 1) must specify 1 or the operation of the processor is UNDEFINED. The exception is taken on the interrupt stack, and IPL is raised to 1F (hex).

The processor pushes a machine check stack frame onto the interrupt stack, consisting of a count longword, an implementation-dependent number of error report longwords, a PC and a PSL. The count longword reports the number of bytes of error report pushed. For example, if 4 longwords of error report are pushed, the count longword will contain 16 (decimal).

Software can decide, on the basis of the information presented, whether to abort the current process if the machine check came from the process. The machine check includes any uncorrected bus and memory errors and any other processor-detected errors. Some processor errors cannot ensure the state of the machine at all. For such errors, the state is preserved as well as possible, given the circumstances.

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