“amoxor.d” instruction does not trigger an exception when accessing an invalid memory address

[fly-1011]

Bug Description:

When executing the amoxor.d a1, s4, (s4) instruction, a discrepancy was observed between the behavior of Spike and NutShell processors. Specifically, when the value of register s4 was 0, the mcause values differed.

According to the RISC-V specification, a Store/AMO Access Fault should occur when performing an atomic operation on an invalid address (in this case, s4 = 0). However, NutShell fails to trigger this fault, resulting in inconsistent behavior compared to Spike.

Spike: mcause = 7, indicating a Store/AMO Access Fault.
NutShell: mcause = 0, meaning no exception was triggered.

Screenshots

[fly-1011]

The information for debugging using spike is as follows: