reenabling interrupts on return from function

[Justin Carlson <justin@cs.cmu.edu>]

I'm obviously missing something basic here.

Looking at stackframe.h, I see this code as a part of RESTORE_SOME


		mfc0	t0, CP0_STATUS;                  \
		.set	pop;                             \
		ori	t0, 0x1f;                        \
		xori	t0, 0x1f;                        \
		mtc0	t0, CP0_STATUS;                  

Here, we're explicitly clearing the IE bit (among others) in the status
register, and we leave it cleared.  The status register is not touched
again until we do an eret.

First, why do we explicitly clear the IE bit, when we're running with
the EXL bit set?  And where is the black magic that is re-enabling
interrupts for the return to usermode?

I must be missing something really fundamental here.  Anyone care to
point out my obvious gaps of knowledge?  :) 

Thanks,
  Justin