each irc normally housed two information retrieval computers, although in some ircs in london just a single machine was present. ircs were generally located within major telephone exchanges, rather than in bt data processing centres, in order to give room for the extensive communications requirements. exchange buildings were ideally suited to housing the large numbers of rack mounted 1200/75 baud modems and associated cabling as well as the racks of 16-port multi-channel asynchronous communications control units (mcaccus) or multiplexors from gec which gave the modems logical access into the computers.