One of the reasons (performance is another, but I think that has been beaten to death) is fault tolerance. Most mainframes are installed in banks and other financial institutions, where one day of downtime can be a major disaster. As far as I know, no x86 or 68xxx processor has ever had any degree of fault tolerance withing the chip, where it is needed. Getting it external to the chip generally involves triplicating everything and having voting circuits, which can be a right royal, pain dealing with the error conditions. Most mainframes, even ones from the 1960s, have error checking throughout the entire system - even in the paths between the registers and ALU. If something goes wrong, like a gate goes into a "stuck at" condition, the redundant circuits and error correctors will jump into action and processing will not stop. Most machines will call home and have replacement parts ordered and engineers busy. For example, a stadard joke (quite true, too) amongst Stratus users is "how do you know when your Stratus has broken? When the new part arrives the next day in FedEx." William Donzelli william(a)ans.net