Mainframe application software development should be listed in Computers/Companies/Software_Development/Mainframes. Service companies also covering AS/400 or other platforms are listed at a higher level, such as Computers/Hardware/Systems/IBM. But service for PCs specifically when attached to mainframes would go here.

Companies that also manufacture IBM mainframe software or freeware are listed instead in Computers/Software/Operating_Systems/Mainframe/IBM/Software.