Secondary bootstrap - entered from M.s in user mode. Its job is to read in
a named file from a specified location, and execute it. Memory management
is set as follows. All kernel I/D space page descriptor registers are set
to 077406 (4Kw, r/w). Kernel D space registers 0 to 5 are set to low memory
(0, 200, etc.). KDA6 is zero (sp is set to 0140000), and KDA7 is 0177600
(I/O page). Kernel I space is placed immediately after the D space
(if separate I/D space), else it coincides with the D space. Memory management
is enabled. The program's BSS segment is cleared, and it is entered by causing
a TRAP to location 034. The new PS (from 036) will show previous mode user.
The Unibus Map is set to point into contiguous memory (from 0), and enabled.

Note that a less-privileged mode can only return to a more privileged mode by:
(1) moving a suitable value directly into the PS (if segmentation allows this);
(2) trapping (if vectors have been set up appropriately).
In particular, "rti" and "rtt" will NOT alter the current or previous modes.
