1. In a processor able to share memory containing data with another processor and having instruction execution means for executing instructions, the improvement comprising: data format signal receiving means for receiving a data format signal from a source external to the processor which provides the data format signal independently of the instructions currently being executed and the data currently being processed by the processor, the data format signal indicating a data format which is an order of pluralities of bytes in the memory employed by the other processor; and operation means in the instruction execution means, the operation means being responsive to data format dependency indicating means in certain ones of the instructions which indicate that the manner in which a read or write operation specified by the instruction is executed depends on the data format employed by the other processor and being also responsive to the data format signal to perform the operation specified by the instruction as required by the data format indicated by the data format signal.