IBM mainframe computers are servers based on the processor instruction set of the System/360 introduced in the mid-1960s, much as today's Intel computers and processors are based on the 8086 used in the original 1981 IBM PC. This category covers the operating systems (OSs) and software products that run on IBM mainframes. Any sites also including mainframe hardware-related information belong in a subcategory of Computers/Hardware/Systems/IBM
. On this page, OSs are arranged in two groups and levels: 1) Top group: issues spanning multiple IBM OSs. 2) Bottom group: specific IBM OSs, individual instances; there is only one OS of this name/type, with its own category.