> So, what do mainframes have that microcomputers don't? Some high-end UNIX servers have historically had many of the features you mention – yet generally not

> So, what do mainframes have that microcomputers don't? Some high-end UNIX servers have historically had many of the features you mention – yet generally not considered "mainframes". Conversely, low-end IBM mainframes (such as the 43xx series) often lacked some of those reliability features – and I'm not sure if all the contemporary non-IBM mainframe platforms have them (especially the ones which are nowadays emulated). I think the real definition of "mainframe" is historical rather than being in terms of any particular technological features. Primarily it refers to high-end commercial-focused (i.e. not supercomputer) computing platforms from before 1980, and their later compatible descendants (which weren't necessarily all that high-end). (That said, "mainframe" and "supercomputer" are historically overlapping categories, since some 1980s supercomputers such as the Hitachi S-3800 were S/370-based and hence arguably belong to both categories; similar remarks could be made about the IBM 3838, or NEC SX-1 and SX-2.) > But the architecture of Unix/Linux isn't channel-oriented Unix and Linux run on IBM mainframes and use IBM mainframe channels for IO. z/Linux uses IBM mainframe channels. So did AIX/370, AIX/ESA, VM/IX, IX/370, and Amdahl UTS. And, z/OS is officially speaking Unix too (it is certified as such by The Open Group) Channels are an implementation detail which usually only OS device drivers need to concern themselves with. (Historically some applications interacted with channels directly – e.g. EXCP on MVS – but IBM strongly discourages that approach nowadays, they'll recommend you use VSAM or DB2 instead, in which all those low-level details are hidden from the application.)