Bigger computers like minicomputers, mainframes, and supercomputers take up large cabinets or even dedicated rooms. These microprocessor based systems were still less costly than time-shared mainframes or minicomputers. Most developers of early mainframes and minicomputers developed similar, but generally incompatible, mail applications. Aldrich's systems were based on minicomputers that could communicate with multiple mainframes. Most companies with mainframes used PCs with the larger computers. Stacks were also used as a basis of a number of mainframes and mini computers. Most other early operating systems for IBM mainframes were also produced by customers. DASD subsystem family which attached to IBM mainframes including the ES/9000 processor family. The company then expanded into disk drives and other peripheral equipment for IBM mainframes. Consequently, MCA usage languished except in IBM's mainframes. MCA was conceptually similar to the channel architecture of the IBM System/360 mainframes. Virtual memory was not available in most IBM mainframes until the System/370 series. STAOPS also produced another communication processor (CP), used in networks hosted by IBM mainframes. SCSS was a software product intended for online use of IBM mainframes. The original releases were for IBM's 7044 & 7090 mainframes.