Analog Devices ADSP-BF53x Blackfin Reference page 608

Instruction Overview
Required Mode
The Idle instruction executes only in Supervisor mode. If execution is
attempted in User mode, the instruction produces an Illegal Use of Pro-
tected Resource exception.
Parallel Issue
This instruction cannot be issued in parallel with other instructions.
Example
idle ;
Also See
System Synchronize
Special Applications
None
16-4 ADSP-BF53x/BF56x Blackfin Processor Programming Reference