|Encoding||Variable (2, 4 or 6 bytes long)|
|Floating point||4 64-bit|
|History of IBM mainframes 1952–present|
|zSeries 900, 800, 990, and 890||z/Architecture|
|zEnterprise System (z196, zEC12)|
ESA/390 (Enterprise Systems Architecture/390) was introduced in September 1990 and was 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 was 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. The 9672 implementations of System/390 were the first high-end IBM mainframe architecture implemented first with CMOS CPU electronics rather than the traditional bipolar logic.
Architecture and Memory 
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 byte-addressable real memory (Central Storage) and Virtual Storage 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 the virtual storage of a single address space cannot exceed 2 GB, ESA/390 supports multiple concurrent 2GB address spaces. Further, each address space can have Dataspaces associated with it, each of which can have up to GB of Virtual Storage. While Central Storage is limited to 2GB additional memory can be configured as expanded storage. With Expanded Storage 4KB pages can be moved between Central Storage and Expanded Storage. Expanded Storage can be used for a number of things such as 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.
In addition, a machine may be divided into Logical Partitions (LPARs), each with its own system memory so that multiple operating systems may run concurrently on one machine.
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, including S/390 itself. A commercial emulation product for IBM xSeries with higher execution speed is also available.
S/390 computers 
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:
|Model||Year Introduced||Number of CPUs||Performance (MIPS)||Memory (GB)|
|G1 – 9672-Rn1, 9672-Enn, 9672-Pnn||1994||1–6||15–66||0.125–2|
|G2 – 9672-Rn2, 9672-Rn3||1995||1–10||15–171||0.125–4|
|G3 – 9672-Rn4||1996||1–10||33–374||0.5–8|
|G4 – 9672-Rn5||1997||1–10||49–447||0.5–16|
|G5 – 9672-nn6||1998||1–10||88–1069||1–24|
|G6 – 9672-nn7||1999||1–12||178–1644||5–32|
In the course of next generations, CPUs added more instructions and increased performance. All 9672s were CMOS, but were slower than the 9021 bipolar machines until the G5 models. CMOS designs permitted much smaller mainframes, such as the Multiprise 3000 introduced in 1999, which was actually based on 9672 G5.
Operating systems 
- http://publib.boulder.ibm.com/cgi-bin/bookmgr/BOOKS/DZ9AR006/1.1?DT=19990630131355 Enterprise Systems Architecture/390 Principles of Operation. IBM Publication No. SA22-7201. Retrieved on 17-09-2007.
- In the context of computer memory, 1 GB = 10243 bytes
- Elliott, Jim (2004-08-17). "The Evolution of IBM Mainframes and VM" (PDF). SHARE Session 9140. Retrieved 2007-10-21. Slide 28: "9672 to zSeries".
- "IBM and Compatible Mainframe Specifications". Technology News of America Co Inc.
- "S/390 Parallel Enterprise Server Announcement". IBM. Retrieved 8 July 2011.
- IBM System/390 Announcement The text of an IBM U.S. Marketing & Services press release distributed on September 5, 1990.
-  Enterprise Systems Architecture/390 Principles of Operation. IBM Publication No. SA22-7201.
|Wikimedia Commons has media related to: IBM S/390|
- Generations of the IBM 360/370/3090/390 by Lars Poulsen with multiple links and references
- Exterior and interior images of the IBM 390 at The Jim Austin Computer Collection, UK Computer Museum. Accessed February 2012
- Porting GCC to the IBM S/390 platform
- IBM Archives: A Brief History of the IBM ES/9000, System/390 AND zSeries