1. A method for exchanging data among a plurality of computer systems by messages passing across a communication path between a computer system A having its own memory and a computer system B also having its own memory, a) writing communication message data that is to be transmitted from computer system A to computer system B into a communication message data area portion of computer system's A own memory which is assigned to be a part of a communication path from computer system A to computer system B; b) setting an indication signal in said computer system B whereby said indication signal corresponds to said communication message data area portion of the memory of computer system A; and c) performing a fetch initiated by computer system B while it only is allowed read access to a remote systems memory, a remote read access to the data in said communication message data area portion of the memory of the remote computer system A for passing from the memory of computer system A the communication message data that was written into the memory of computer system A to the computer system B and fetched from said communication message data area portion of said computer system A's own memory by said remote read access from computer system B; and further comprising the steps of: after setting an indication signal as a vector which can be mapped to the start address of a communication area portion of computer system B's own memory in a latch section assigned to a communication path from the computer system A to computer system B to indicate that there is a message stored in said communication message data area portion of said computer system A's own memory ready to be sent from computer system A to computer system B whereby said indication signal vector corresponds to said communication message data area portion of the memory of computer system A, an interrupt is initiated at computer system B as soon as said indication signal has been set in said latch section, said interrupt causing a microcode routine to perform a translation to map the interrupt information to the memory address of the corresponding communication message data area in communication system A; and wherein said latch section assigned to a communication path from the computer system A to computer system B to indicate that there is a message stored in said communication message data area portion of said computer system A's own memory ready to be sent from computer system A to computer system B and into which said indication signal vector is set and interrupt taken, has a busy portion and a request portion, and said exchange further comprising the steps of: before said data exchange is started by setting an indication signal in said computer system B whereby said indication signal corresponds to said communication message data area portion of the memory of computer system A, said busy portion controls by its active setting whether or not said request portion can be set for data exchange, and after said request portion is set with said vector in said latch section, said computer system B generates a remote fetch in order to access the corresponding communication message data area portion of said computer system A's own memory whereupon the contents thereof are transferred into a communication messages data area portion of said computer system B's own memory, and as soon as this data is completed said computer system B signals the successful arrival in its communication message data area, and reads the contents thereof into its computer system B's own memory, and it then resets said request portion in said latch section to indicate completion of the data exchange, and further comprising the steps of: after the contents of the message exchanged between computer system A and computer system B have been received and the request portion in the latch section has been reset, said computer system B interprets the message and starts the requested task, and if no immediate response message is to be issued, said computer system B also resets the busy portion of the latch section to allow usage of the just used communication path between the computer system A and computer system B again for message passing.