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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.
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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.
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Designer | IBM |
---|---|
Bits | 32-bit |
Introduced | 1988 |
Design | CISC |
Type | Register–Register Register–Memory Memory–Memory |
Encoding | Variable (2, 4 or 6 bytes long) |
Branching | Condition code, indexing, counting |
Endianness | Big |
Predecessor | System/370-XA |
Successor | ESA/390 |
Registers | |
General-purpose | 16 |
Floating point | 4 64-bit |
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On February 15, 1988, IBM announced[6][7] 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.[8] 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.
Designer | IBM |
---|---|
Bits | 32-bit |
Introduced | 1990 |
Design | CISC |
Type | Register–Register Register–Memory Memory–Memory |
Encoding | Variable (2, 4 or 6 bytes long) |
Branching | Condition code, indexing, counting |
Endianness | Big |
Predecessor | ESA/370 |
Successor | z/Architecture |
Registers | |
Access 16× 32, Control 16×32, Floating Point Control (FPC) 32-bit, Prefix 32 bit, PSW 64-bit | |
General-purpose | 16 |
Floating point | 4 64-bit up to the G4; 16 64-bit starting with the G5[9][10] |
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.[11][12]
Starting with the System/390 G5,[9][10] IBM introduced:[13]
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.
ESA/390 adds the following[14] facilities
The following channel commands[lower-alpha 6] are new, or have their functionality changed, in ESA/390:[12]
Command | Bit Position | |||||||
---|---|---|---|---|---|---|---|---|
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | |
Read configuration data | D | D | D | D | D | D | D | 0 |
Read node identifier | D | D | D | D | D | D | D | 0 |
Sense ID | 1 | 1 | 1 | 0 | 0 | 1 | 0 | 0 |
Set interface identifier | D | D | D | D | D | D | D | 1 |
Note: D Device dependent. The command code, if any, recognized by an I/O device may be obtained by using a sense-ID command. |
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