Interrupt disabled in ISR

[Leon Pollak]

Hello,
	I shall be very thankful for the explanation of the following moment:
In the PPC code file irq_stub.S routine __ISR_handler I see that all 
interrupts are disabled almost immediatly. They are reenabled at the end of 
the ISR, after the user ISR has finished its processing, almost at the point 
of exit.
	The text contains the comment after the _ISR_NestLevel incrementing:
"From here on out, interrupts can be re-enabled, RTEMS convention says not".
	Why?

Thanks.
-- 
leonp at plris.com