The IBM System/390 is a discontinued mainframe product family implementing ESA/390, the fifth generation of the System/360 instruction set architecture.

The 9221 processors were made of VLSI CMOS chips designed in Böblingen, Germany, whence the 9672 line later originated.

The lower 6 of the 8 water-cooled models (codenamed H0) were immediately available, but used the same processor as the 3090-J, still at the 69 MHz (14.5 ns) maximum frequency and thus with unchanged performance.

Those models' main difference from the 3090-J was the optional addition of ESCON, Sysplex and Integrated Cryptographic Feature.

Only the models 900 and 820 had an all-new design (codenamed H2),[c] featuring private split I+D 128+128 KB L1 caches and a shared 4 MB L2 cache (2 MB per side) with 11-cycle latency, more direct interconnects between the processors, multi-level TLBs, branch target buffer and 111 MHz (9 ns) clock frequency.

However unlike the old S/360-91-derived systems, the models 900 and 820 had full out-of-order execution for both integer and floating-point units, with precise exception handling, and a fully superscalar pipeline.

These models, codenamed H5, had double the L2 cache and 30% higher per-processor performance than the H2 line, and added a hardware data compression.

[37][38][39] Previously available only on IBM 3090, Logical Partitions (LPARs) are a standard feature of the ES/9000 processors whereby IBM's Processor Resource/Systems Manager (PR/SM) hypervisor allows different operating systems to run concurrently in separate logical partitions (LPARs), with a high degree of isolation.

[53] This was introduced as part of IBM's moving towards "lights-out" operation and increased control of multiple system configurations.

[67][68] The thousandth G5 system shipped less than 100 days after the manufacturing began; the greatest ramping of production in S/390's history.

[69] In late May 1999 the G6 arrived featuring copper interconnects, raising the frequency to 637 MHz, higher than the fastest DEC machines at the time.

It emerged from a line of S/390-compatibility/coprocessor cards for PCs, but is a true S/390 system capable of server duties, having relegated the Pentium II to the role of an I/O coprocessor.