Nested spl's really needed? - (nf)

[emrath at uiuccsb.UUCP]

#R:ccvaxa:15200007:uiuccsb:14900001:000:1201
uiuccsb!emrath    Jul  1 18:16:00 1983

  The question about nested interrupt levels prods me to ask a
related question that I have wondered about before.
Can somebody explain the following code in "locore.s" at "alltraps:"?

alltraps:
	....
	mtpr	$HIGH,$IPL	# <<< WHY IS THIS HERE???
	rei

  This mtpr instruction raises the priority level to the max, so that the rei
won't fail by trying to go to a higher priority level. When I first read that
the rei instruction checked for this, I thought it was really smart, for it
helps the system programmer find buggy code which improperly handles the
priority level. Any interrupt/exception routine that LOWERS the priority
below the interrupted level had better know damn well what it is doing.
I've seen systems that would crash or hang once every 3.2 blue moons, only
to find that some interrupt routine was lowering the priority, allowing
re-entry or corruption of data structures (possibly of some unrelated device
handler).
  The mtpr instruction should not be there. Any routine that lowers the
priority should explicitly raise it again, and the reason for
doing so should be WELL commented. 

		Perry Emrath, Univ. of IL
		...{decvax|inuxc}!pur-ee!uiucdcs!emrath
		emrath.uiuc at rand-relay