Page "Z/OS" Paragraph 4

From its inception z / OS has supported tri-modal addressing ( 24-bit, 31-bit, and 64-bit ).

Up through Version 1. 5, z / OS itself could start in either 31-bit ESA / 390 or 64-bit z / Architecture mode, so it could function on older hardware albeit without 64-bit application support on those machines.

( Only the newer z / Architecture hardware manufactured starting in the year 2000 can run 64-bit code.

) IBM support for z / OS 1. 5 ended on March 31, 2007.

Now z / OS is only supported on z / Architecture mainframes and only runs in 64-bit mode.

z / Architecture hardware always starts running in 31-bit mode, but current z / OS releases quickly switch to 64-bit mode and will not run on hardware that does not support 64-bit mode.

Application programmers can still use any addressing mode: all applications, regardless of their addressing mode ( s ), can coexist without modification, and IBM maintains an unwavering commitment to tri-modal backward compatibility.

However, increasing numbers of middleware products and applications, such as DB2 Version 8 and above, now require and exploit 64-bit addressing.