"There is a history of mainframe Unix ports that goes back many years. IBM's AIX/390 and AIX/ESA are both now defunct, but Amdahl's UTS and Hitachi's OSIRIS are alive and kicking, and still selling.

The hardware architecture in IBM's Z mainframes. z/Architecture was an evolutionary step beyond ESA/390. The primary difference is the widening of all registers from 32 to 64 bits and the use of ...