HLT-Instruction

I have tried to implement a new idle thread that uses inline assembler
to execute the hlt-Instruction (slowdown the system to become cooler
if idle). Ok, I have received the information that it is not allowed
for a normal user thread (ring3) to execute hlt. Must be on ring0
(within a ISR). I have tried this (Timer-Ticker ISR), but this won’t
work, the system crashes.
Is there any idea how to implement/use “hlt”?