How does the kernel detect a bus check HPMC?

[mvpel]

Dennis - some of the PCI cards in the system have PROT_URW IO maps so that they can be used within critical segments of real-time code where system calls are not allowed - it was this fact in combination with LIBCL's dereference of an uninitialized pointer that caused the HPMC.

We're given to understand that HP's intent was that the previous frame descriptor's structure would have a calloc() before the first call to U_get_previous_frame(), but that intent didn't make it into the documentation nearly 15 years ago.