z/Architecture - technical definition

The hardware architecture in IBM's zSeries (mainframes). z/Architecture is an evolutionary step beyond ESA/390. The primary difference is the widening of all registers from 32 to 64 bits and the use of 64-bit addressing for both real and virtual storage. It includes the ability to execute programs using older 24 and 31-bit addressing so that ESA/390 programs run unchanged.

z/Architecture adds 130 instructions, many of which deal with 64-bit data, and support for Variable Workload License Charges, which allows software to be priced based on usage. See z/OS.