IBM e(logo)server zSeries is the name for the System 390 (S/390) family since December 2001,the then current models being rebranded as the zSeries 900 family. They are the successors to the System/370s and more specifically the System 370/ESAs.
IBM e(logo)server zSeries 800 family :
- is using z/Architecture (64-bit real and virtual addresses).
- it offers up to 640-processors in a 32 way Sysplex (512 performing workloads and up to 128 performing system and backup functions).
- it supports the Linux, TPF, z/OS and z/VM and operating systems.
- S/390 ESA applications are fully compatible with z/Architecture.
In May 2003
,IBM announced 
the zSeries 990 family
,which began shipping at the end of that October.These models promised nearly three times the total system capacity,four times the memory,and over four times the I/O and networking bandwidth of the z900 family.
The zSeries z990 Model D32 is currently the most powerful IBM mainframe
computer,capable of over 9,000,000,000 instructions per second.