The behaviour of Arm instructions is specified using a pseudocode language called ASL that does not yet have semantics. We present an interpreter for this language in both sequential and concurrent setting. This allows us to compare the behaviours of AArch64 instructions as implemented by herd7 , the tool which Arm uses for describing its memory model, against the behaviours of the same instructions as described in ASL. We find discrepancies between those semantics for the Compare-And-Swap instruction.