Computer system with two busses

In a computer system having a plurality of units interconnected by first and second common busses, said units communicating with one another by means of messages, at least one of such messages being a requesting message which includes a command requiring a reply message and the addresses of the sending and receiving units, the computer system comprising:

• first, second, third and fourth ones of the units each including message transmitting means and message receiving means, the third and forth units further including means for processing the commands of such requesting messages and for providing a ready to return message signal upon the completion of such processing;
• means for coupling the message transmitting means and message receiving means of the first and second units to the first and second busses, respectively, and the message transmitting and message receiving means of the third and fourth units to the second and first busses, respectively;
• bus control means including
• first means for selectively enabling the message transmitting means of the first and second units to send messages including requesting messages over the first bus during first and second bus transfer intervals such that the message receiving means of the third and fourth units receive such requesting messages containing their respective receiving unit addresses;
• and
• second means operable in response to the ready to return message signals for selectively enabling the message transmitting means of the third and fourth units to return reply messages over the second bus during said first, second or other bus transfer intervals in accordance with the time sequence of such signals and in accordance with a priority schedule when two of such signals compete for the same bus transfer interval.