Tie-breaking control circuit for bus modules which share command

Abstract

In a data processing system, a command sending module sends a command over a bus to two command executing modules at the same time, and that command is to be performed by either one, but not both, of the command executing modules. To ensure that only one of the command executing modules executes the command, they each include a tie-breaking control circuit which is comprised of: a) a storage circuit that initially stores a first control code upon the receipt of the command; b) a bus acquisition circuit which requests and subsequently obtains the use of the bus in response to the receipt of the command; c) a write circuit which changes the first control code to a second control code in the storage circuit if an acceptance message is received from the other command executing module between requesting and obtaining the bus; and, d) a conditional message sending circuit which, while the bus is obtained, senses the content of the storage circuit and sends the acceptance message on the bus only if the sensed content is the first control code.