Illegal mode change handling

摘要

A data processing system 2 supporting multiple modes of operation is provided with illegal change detecting circuitry 22 which detects attempts by program instructions to perform an illegal change of mode, such as a change to a higher level of privilege in response to execution of a mode changing program instruction or an exception return instruction. If such a change is detected, then an illegal change bit CPSR.IL is set. An instruction decoder 12 is responsive to the illegal change bit having a set value to treat subsequent program instructions as undefined instructions. These undefined instructions may then trigger an undefined instruction exception or other type of response.

主权项

1. An apparatus for processing data comprising:
processing circuitry responsive to program instructions to perform data processing operations, said processing circuitry having a plurality of modes, said plurality of modes include at least one of a plurality of different instruction set modes, each instruction set mode corresponding to decoding and execution of program instructions from a different instruction set; and illegal change detecting circuitry responsive to an attempt to perform an illegal change of mode of said processing circuitry specified by an illegal program instruction: (i) to suppress said illegal change of mode; and (ii) to set an illegal change bit to a predetermined value indicative of said illegal program instruction attempting to perform said illegal change; wherein while said illegal change bit has said predetermined value, said processing circuitry responds to further program instructions as undefined instructions.