Is there any Guru who knows why interrupts are disabled in ISR?

[Leon Pollak]

Hello Gurus,
        I shall be very thankful for the explanation of the following moment:
        I look into 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 from __ISR_handler.
        This text contains the comment after the _ISR_NestLevel incrementing:
"From here on out, interrupts can be re-enabled, RTEMS convention says not".
        Can somebody explain me why interrupts must be disabled inside the 
user ISR?

Thanks.
-- 
l
-- 
Dr.Leon M.Pollak
Director
PLR Information Systems Ltd.
leonp at plris.com