IBM Enterprise Systems Architecture

IBM Enterprise Systems Architecture is an instruction set architecture 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 models by adding a new mode in which general-purpose registers 1-15 are each associated with an access register 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 floating-point operations and increased the number of floating-point registers from 4 to 16.

Enterprise Systems Architecture is essentially a 32-bit 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 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 dating back to 1964 with the 24-bit-address/32-bit-data (System/360 and System/370) 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 addition 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 access register 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.

A machine may be divided into Logical Partitions (LPARs), each with its own virtual system memory so that multiple operating systems may run concurrently on one machine.

ESA/390 architecture
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.

Starting with the System/390 G5, IBM introduced:
 * the basic floating-point extensions facility, which increases the number of floating-point registers from 4 (0, 2, 4, 6) to 16 (0-15);
 * the binary floating-point (BFP) extensions facility, which supports IEEE 754 binary floating-point numbers, with an additional floating-point control (FPC) register to support IEEE 754 modes and errors;
 * the floating-point support (FPS) extensions facility, which adds instructions to load and store floating-point numbers regardless of whether they're in hexadecimal or IEEE 754 format and to convert between those formats;
 * the hexadecimal floating-point (HFP) extensions facility, which adds new hexadecimal floating-point instructions corresponding to some binary floating-point instructions.

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 facilities
ESA/390 adds the following facilities


 * All models
 * Access-list-controlled protection


 * Some models
 * Concurrent sense
 * PER 2
 * Storage-protection override
 * Move-page facility 2
 * Square root
 * String instruction
 * Suppression on protection with virtual-address enhancement
 * Set address space control fast
 * Subspace group
 * Called-space identification
 * Checksum
 * Compare and move extended
 * Immediate and relative instruction
 * Branch and set authority
 * Perform locked operation
 * Additional floating-point
 * Program call fast
 * Resume program
 * Trap
 * Extended TOD clock
 * TOD-clock-control override
 * Store system information
 * Extended translation 1
 * Extended translation 2
 * z/Architecture (certain instructions)
 * Enhanced input/output

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