Nice

[terryl at sail.LABS.TEK.COM]

In article <2007 at necisa.ho.necisa.oz.au> boyd at necisa.ho.necisa.oz.au (Boyd Roberts) writes:
+In article <1991Jan31.180957.9167 at turnkey.tcc.com> jackv at turnkey.TCC.COM (System Administrator) writes:
+>... But it is also not correct that a context
+>switch will take as long as you suggest. What will happen is that the
+>interrupt will cause the kernel to run in trap(),
+
+Device interrupts do not go through trap().  The stack is munged, the
+interrupt routine is called, the stack un-munged and the interrupt is
+returned from.  No calls to trap().  trap() handles exceptions, not
+device interrupts.


     `Tis true (that device interrupts do not go through trap()), but in
almost all modern Unices I've seen, there is a check just right before
returning from the interrupt(and it goes like this, in my best Ray Davies
imitation!!! (-:):

	If we were interrupted in USER mode
			AND
	We need to reschedule the cpu
	then call trap() to reschedule the cpu....

__________________________________________________________
Terry Laskodi		"There's a permanent crease
     of			 in your right and wrong."
Tektronix		Sly and the Family Stone, "Stand!"
__________________________________________________________