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R15 is unpredictable as a source or destination in most data processing operations. R15 is also unpredictable as a transfer register in certain load/store instructions. Examples of such instructions include LDRT, LDRH, and LDRB.
In the Cortex®‑M33 processor, the use of R15 as a named register specifier for any source or destination register that is indicated as unpredictable generates an UNDEFINSTR UsageFault exception.