At the high end are the operating systems for the main frames, those room sized computers still found in major corporate data centers. These computers distinguish themselves from personal computers in terms of their I/O capacity. A main frame with 1000 disks and thousands of gigabytes of data is not unusual; a personal computer with these specifications would be odd indeed. Mainframes are also making something of a comeback as high end. Web servers, servers for large-scale electronic commerce sites, and servers for business-to-business transactions.
The operating systems for mainframes are heavily oriented toward processing many jobs at once, most of which need prodigious amounts of I/O. they typically offer three kinds of services: batch, transaction processing, and timesharing. These factions are closely related: mainframe operating systems often perform all of them .An example mainframe operating system is OS/390, a descendant of OS/360