The hardware architecture in IBM's System z (zSeries) 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 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. Introduced in 2000, z/Architecture added 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 zEnterprise, System z and z/OS.