Build a kernel with DDB and DEBUG. Add the following two patches to trap.c: 1. In the initial test "if (trapdebug)", add a delay(5000000) or so. (Also turn on trapdebug.) 2. In the "case T_PAGEFLT|T_USER:", after "vm" is set but before it is referenced add: if ((int)vm == 0) goto we_re_toast;