IBM Enterprise Systems Architecture
IBM Enterprise Systems Architecture is an instruction set architecture introduced by IBM as Enterprise Systems Architecture/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 Enterprise Systems Architecture/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 binary 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 and Virtual Storage addressing is limited to 31 bits, as is the case with 370-XA. It maintains problem state backward compatibility dating back to 1964 with the 24-bit-address/32-bit-data and subsequent 24/31-bit-address/32-bit-data architecture. However, the I/O subsystem is based on System/370 Extended Architecture, not on the original S/370 I/O instructions.
ESA/370 architecture
On February 15, 1988, IBM announcedEnterprise Systems Architecture/370 for 3090 enhanced 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 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, 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 to 16 ;
- the binary floating-point extensions facility, which supports IEEE 754 binary floating-point numbers, with an additional floating-point control register to support IEEE 754 modes and errors;
- the floating-point support 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 extensions facility, which adds new hexadecimal floating-point instructions corresponding to some binary floating-point instructions.
New facilities
ESA/390 adds the following facilities;All models
;Some models