With high-end operating system for the main frame, the room size computers are still found in large corporate data centers. The computer distinguish themselves from the personal computer in terms of their I / O capacity. A main frame with 1000 disks and thousands of gigabytes of data is extraordinary, a personal computer with details are strange indeed. Mainframes are also making something of a comeback as the high end. Web servers, servers for large-scale electronic commerce sites, and servers for business-to-business transactions.

The operating system for mainframes is heavily oriented toward processing multiple jobs at once, most of which need huge amounts of I / O. These generally offer three types of services: batch, transaction processing and timesharing. The factions are closely related. mainframe operating systems tend to perform all of these systems is an example of mainframe OS/390 operating is, a descendant of OS/360.