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ESA/390 (Enterprise Systems Architecture/390) has been introduced in the September 1990^[1] and is IBM's last 31-bit-address/32-bit-data mainframe computing design, copied by Amdahl, Hitachi, and Fujitsu among other competitors. It was the successor of System/370 and has been succeeded by the 64-bit z/Architecture in 2000.

Machines supporting the architecture have been sold under the brand System/390 (S/390) from the beginning of the 1990s. It was the only IBM mainframe architecture implemented first with bipolar and later with CMOS CPU electronics.

The architecture employs a channel I/O subsystem in the System/360 tradition, offloading almost all I/O activity to specialized hardware in the mainframe tradition.

The architecture maintained backward compatibility with the 24-bit-address/32-bit-data System/360 (1964) and all intermediate large system 24/31-bit-address/32-bit-data architectures (System/370, System/370-XA, and ESA/370).

ESA/390 is arguably a 32-bit architecture; as with System/360, System/370, 370-XA, and ESA/370, the general-purpose registers are 32 bits long, and the arithmetic instructions support 32-bit arithmetic. Only memory addressing is limited to 31 bits. (IBM reserved the most significant bit to easily support applications expecting 24-bit addressing, as well as to sidestep a problem with extending two instructions to handle 32-bit unsigned addresses.)

In fact, total system memory is not limited to 31 bits (2 GB). While a single address space cannot exceed 2 GB, ESA/390 supports multiple concurrent address spaces. These address spaces are dedicated to different LPARs of the machine and system memory areas larger than 2GB can be configured as expanded storage and dedicated to an LPAR, where 4k pages from expanded storage can be copied into main storage and reverse. Thus, such memory can be used for ultra-fast paging, for disk caching and virtual disks within the VM/CMS operating system. Under Linux/390 this memory cannot be used for disk caching, instead it is supported by a block device driver, allowing to use it as ultra-fast swap space and for ram disks.

An important capability to form a Parallel Sysplex was added to the architecture in 1994.

Some PC-based IBM-compatible mainframes which provide ESA/390 processors in smaller machines have been released over time, but were only intended for software development.

The Hercules emulator is a portable ESA/390 and z/Architecture machine emulator which supports enough devices to boot many ESA/390 operating systems. Since it is written in pure C, it has been ported to many platforms. A commercial emulation product for IBM xSeries with higher execution speed is also available.

The ESA/390 architecture was introduced with IBM ES/9000 family of mainframes.

Later, since 1994, the IBM 9672 machines were the largest and most notable. This line has been built in 6 hardware generations:^[2]

• G1 – 9672-Rn1, 9672-Enn, 9672-Pnn
• G2 – 9672-Rn2, 9672-Rn3
• G3 – 9672-Rn4
• G4 – 9672-Rn5
• G5 – 9672-nn6
• G6 – 9672-nn7

In the course of next generations, CPUs added more instructions and were turned from bipolar to CMOS. G1 and G2 were bipolar, G3 and G4 were the first CMOS generations, which were slower than the bipolar models. CMOS caught up in the G5 and G6 generations. CMOS designs permitted much smaller mainframes, such as the Multiprise 3000 introduced in 1999, which was actually based on 9762 G5.

OS/390, VM/CMS, VSE, Linux and all systems supported by System/370.