The IBM eserver zSeries from International Business Machines is the name for the IBM System/390 mainframess since December 2001; the then current S/390 models being rebranded as the zSeries 900. They succeed the System/370, more specifically the S/370 ESA.

Features of the eserver zSeries 900 family:

• Based on z/Architecture (64-bit real and virtual addresses)
• Offers up to 640 processors in a 32-way Sysplex (512 processors performing workloads and up to 128 for system and backup functions)
• Supports the Linux, TPF, z/OS and z/VM operating systems
• S/390 ESA applications are fully compatible with z/Architecture

In May 2003, IBM announced [1] the zSeries 990 family, which began shipping at the end of that October. These models promises 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 990 Model D32 is currently the most powerful IBM mainframe computer available, reportedly capable of performing more than 9,000,000,000 instructions per second.

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