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Pub. No.:    WO/2017/149269    International Application No.:    PCT/GB2017/050344
Publication Date: Sat Sep 09 01:59:59 CEST 2017 International Filing Date: Sat Feb 11 00:59:59 CET 2017
IPC: G06F 9/38
Applicants: ARM LIMITED
Inventors: PARKER, Jason
A data processing system 2 operates at a plurality of exception levels ELx and supports the use of protected execution environments. A register bank 16 contains registers having associated ownership variables indicating an owning exception level. Register access control circuitry 30 is responsive to the ownership values for respective registers to control access to those registers by processing circuitry 14 in dependence upon the ownership values. Target-constrained data transfer operations and associated program instructions may be provided which are able to access data values in registers not owned by the exception level associated with the execution of those program instructions, but are limited to perform data transfers to or from memory locations within a memory 6 indicated by an architected storage pointer for the owning exception level. Target-unconstrained transfer instructions at a given exception level are not able to access register data value marked as owned by a different exception level.