A compare and swap transaction can be issued by a master device to request a processing unit to select whether to write a swap data value to a storage location corresponding to a target address in dependence on whether a compare data value matches a target data value read from the storage location. The compare and swap data values are transported within a data field of the compare and swap transaction. The compare data value is packed into a first region of the data field in dependence of an offset portion of the target address and having a position within the data field corresponding to the position of the target data value within the storage location. This reduces latency and circuitry required at the processing unit for handling the compare and swap transaction.