- The available processor modes vary by ARM model.
- The purpose of processor modes is to regulate access to memory and hardware resources so that a process initiated by a specific user can’t access the memory of other processes or access hardware they don’t have permission for
- Most ARM processors have two security levels for processes. PL0 is for user mode programs and then PL1 is for operating system code. Newer ARM processors used in servers have a third level PL2 for virtualization hypervisors, so they can keep their various hosted operating systems completely separate. There is also an optional ARM build for secure computing, if this is present then there is an even higher PL3 level that is used for a system security monitor.
https://developer.arm.com/documentation/den0013/d/ARM-Processor-Modes-and-Registers
https://smist08.wordpress.com/2019/12/02/arm-processor-modes/