IBM Enterprise Systems Architecture

ESA/390
Designer	IBM
Bits	32-bit
Introduced	1990; 34 years ago
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
General-purpose	16
Floating point	4 64-bit up to the G4; 16 64-bit starting with the G5

*IBM S/370-ESA and S/390-ESA registers*
	;
	;
	;
	;
	;
	;
	;
	;
	;
	;
20	Fixed-point overflow
21	Decimal overflow
22	Exponent underflow
23	Significance

ESA/370
Designer	IBM
Bits	32-bit
Introduced	1988; 36 years ago
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

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^[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.

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.^[11]^[12]

Starting with the System/390 G5,^[9]^[10] IBM introduced:^[13]

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^[14] 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^[f] are new, or have their functionality changed, in ESA/390:^[12]

ESA/390 I/O-Device Commands
Command	Bit Position
Command	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.

Notes

References

S370-ESA: IBM Enterprise Systems Architecture/370 Principles of Operation (PDF) (First ed.). IBM. August 1988. SA22-7200-0.
S/390-ESA: IBM Enterprise Systems Architecture/390 Principles of Operation (PDF) (Ninth ed.). IBM. June 2003. SA22-7201-08.

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