IBM Enterprise Systems Architecture is an

instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...

introduced by IBM as ESA/370 in 1988. It is based on the IBM System/370-XA architecture. It extended the dual-address-space mechanism introduced in later

IBM System/370 The IBM System/370 (S/370) is a model range of IBM mainframe computers announced on June 30, 1970, as the successors to the System/360 family. The series mostly maintains backward compatibility with the S/360, allowing an easy migration path f ...

models by adding a new mode in which general-purpose registers 1-15 are each associated with an

access register In IBM terminology, an Access Register (AR) is a hardware register in ESA/370 and later processors. Access registers work in conjunction with the general purpose registers, giving a program transparent access to up to sixteen 2 GB address ...

referring to an address space, with instruction operands whose address is computed with a given general-purpose register as a base register will be in the address space referred to by the corresponding address register. The later ESA/390, introduced in 1990, added a facility to allow device descriptions to be read using channel commands and, in later models, added instructions to perform

IEEE 754 The IEEE Standard for Floating-Point Arithmetic (IEEE 754) is a technical standard for floating-point arithmetic established in 1985 by the Institute of Electrical and Electronics Engineers (IEEE). The standard addressed many problems found i ...

floating-point operations and increased the number of floating-point registers from 4 to 16. Enterprise Systems Architecture is essentially a

32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calcula ...

architecture; as with System/360, System/370, and 370-XA, 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, as is the case with 370-XA. (IBM reserved the

most significant bit In computing, bit numbering is the convention used to identify the bit positions in a binary number. Bit significance and indexing In computing, the least significant bit (LSB) is the bit position in a binary integer representing the binary ...

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.) It maintains problem state

backward compatibility Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially ...

back to 1964 with the

24-bit Notable 24-bit machines include the CDC 924 – a 24-bit version of the CDC 1604, CDC lower 3000 series, SDS 930 and SDS 940, the ICT 1900 series, the Elliott 4100 series, and the Datacraft minicomputers/Harris H series. The term SWORD i ...

-address/32-bit-data (

System/360 The IBM System/360 (S/360) is a family of mainframe computer systems that was announced by IBM on April 7, 1964, and delivered between 1965 and 1978. It was the first family of computers designed to cover both commercial and scientific applic ...

and

System/370 The IBM System/370 (S/370) is a model range of IBM mainframe computers announced on June 30, 1970, as the successors to the System/360 family. The series mostly maintains backward compatibility with the S/360, allowing an easy migration path f ...

) and subsequent 24/31-bit-address/32-bit-data architecture ( System/370-XA). However, the I/O subsystem is based on System/370 Extended Architecture (S/370-XA), not on the original S/370 I/O instructions.

ESA/370 architecture

On February 15, 1988, IBM announced Enterprise Systems Architecture/370 (ESA/370) for 3090 enhanced ("E") models and for 4381 model groups 91E and 92E. In additional to the primary-space and secondary-space addressing modes that later System/370 models, and System/370 Extended Architecture (S/370-XA) models, support, ESA has an

mode in which each use of general register 1-15 as a base register uses an associated access register to select an address space. In addition to the normal address spaces that machines with the dual-address-space facility support, ESA also allows data spaces, which contain no executable code.

ESA/390 architecture

A machine may be divided into Logical Partitions (

LPAR A logical partition (LPAR) is a subset of a computer's hardware resources, virtualized as a separate computer. In effect, a physical machine can be partitioned into multiple logical partitions, each hosting a separate instance of an operating ...

s), each with its own virtual 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. ESA/390 also extends the Sense ID command to provide additional information about a device, and additional device-dependent channel commands, the command codes for which are provided in the Sense ID information, to allow device description information to be fetched from a device. Some PC-based IBM-compatible mainframes which provide ESA/390 processors in smaller machines have been released over time, but are only intended for software development.

New channel commands

The following channel commands are new, or have their functionality changed, in ESA/390:

Notes

References

:;S370-ESA :: :;S/390-ESA :: {{reflist Enterprise Systems Architecture Computing platforms Computer-related introductions in 1988 2000s disestablishments 32-bit computers