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If RS1 is X0 or if using an immediate and the value encoded in the RS1 field which is used as the immediate is zero, then the CSR is not actually written. It doesn't matter whether the register value would change or not. Also, if an instruction wants to write to the CSR, the manual does not say anything about whether or not the register value changes. If a register is read only, attempting to write it with *any* value should be illegal. Change-Id: Ic876eff0fc0c7e25a40ba9213e3c3e49bfa7a33e Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45522 Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu> Maintainer: Gabe Black <gabe.black@gmail.com> Tested-by: kokoro <noreply+kokoro@google.com>