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

ESA/370
DesignerIBM
Bits32-bit
Introduced1988 (1988)
DesignCISC
TypeRegister–Register
Register–Memory
Memory–Memory
EncodingVariable (2, 4 or 6 bytes long)
BranchingCondition code, indexing, counting
EndiannessBig
PredecessorSystem/370-XA
SuccessorESA/390
Registers
General-purpose16
Floating point4 64-bit
IBM S/370-ESA and S/390-ESA registers
General Registers 0-15

Two's complement value
0 31
Access Registers 0-15[1]

0 0 0 0 0 0 0 P ALESN ALEN

0 6 7 8 15 16 31
ESA Access register abbreviations
Bits Field Meaning
0-6 0000000
7 P Primary
0=use dispatchable-unit access list
1=use primary-space access list
8-15 ALESN access-list-entry sequence number
16-31 ALEN access-list-entry number
Control Registers 0-15

See Principles of Operation[2][3] or Control Registers
0 31
Floating Point Registers 0-6/0-15[lower-alpha 1]

S Biased exponent Mantissa
0 1 7 8 31

Mantissa (continued)
32 63
Enterprise Systems Architecture Extended Control mode PSW [4] [5]

0 R 0 0 0 T I
O		 E
X		 Key 1 M W P AS CC Program
Mask		 0 0 0 0 0 0 0 0
0 1 2 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 31

A Instruction Address
32 33 63
ESA EC mode PSW abbreviations
Bits Field Meaning
1 R PER Mask
5 T DAT mode
6 IO I/O Mask; subject to channel mask in CR2
7 EX External Mask; subject to external subclass mask in CR0
8-11 Key PSW key
12 E=1 Extended Control mode
13 M Machine-check mask
14 W Wait state
15 P Problem state
16-17 AS Address-Space Control
00=primary-space mode
01=Access-register mode
10=Secondary-space mode
11=Home-space mode
18-19 CC Condition Code
20-23 PM
Program Mask
Bit Meaning
20 Fixed-point overflow
21 Decimal overflow
22 Exponent underflow[lower-alpha 2]
23 Significance[lower-alpha 3]
32 A Addressing mode
0=24 bit; 1=31 bit
33-63 IA Instruction Address

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.

ESA/390 architecture

ESA/390
DesignerIBM
Bits32-bit
Introduced1990 (1990)
DesignCISC
TypeRegister–Register
Register–Memory
Memory–Memory
EncodingVariable (2, 4 or 6 bytes long)
BranchingCondition code, indexing, counting
EndiannessBig
PredecessorESA/370
Successorz/Architecture
Registers
General-purpose16
Floating point4 64-bit up to the G4; 16 64-bit starting with the G5[9]

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.

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.[10]: 1-8 [11]

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[lower-alpha 4] are new, or have their functionality changed, in ESA/390:[11]

ESA/390 I/O-Device Commands
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.

Notes

  1. ↑ The number and format of floating-point registers depends on the installed features:
    ESA/370
    ESA/390 without the Advanced Floating Point (AFP) facility
    Only the hexadecimal floating point (HFP) registers FP0, FP2, FP4 and FP6 exist
    ESA/390 with the AFP facility
    FP0_FP15 may be HFP or IEEE floating point
  2. ↑ Bit 22 is renamed as HFP exponent underflow in ESA/390
  3. ↑ Bit 23 is renamed as HFP significance in ESA/390
  4. ↑ The data returned by Sense ID include the command codes for Read configuration data, Read node identifier and Set interface identifier.

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.
  1. ↑ S370-ESA, p. 5-30, Access-list-Entry Token.
  2. ↑ S370-ESA, pp. 4-7–4-9, Figure 4-3 Assignment of Control-Register Fields.
  3. ↑ S390-ESA, pp. 4-8–4-10, Figure 4-3 Assignment of Control-Register Fields.
  4. ↑ S370-ESA, p. 4-5, Program-Status-Word Format.
  5. ↑ S390-ESA, p. 4-5, Program-Status-Word Format.
  6. ↑ "IBM 3090 PROCESSOR UNIT MODELS 280E AND 500E AND IBM 3090 PROCESSOR UNIT MODEL 300E TO 400E UPGRADE". Announcement Letters. IBM. February 15, 1988. 188-038.
  7. ↑ "ENTERPRISE SYSTEMS ARCHITECTURE/370 (TM) AND MVS/SYSTEM PRODUCT VERSION 3". Announcement Letters. IBM. February 15, 1988. 288-059.
  8. ↑ S370-ESA.
  9. ↑ Slegel, Timothy J. (August 17, 1998). "IBM S/390 G5 Microprocessor" (PDF). Hot Chips.
  10. ↑ S390-ESA.
  11. 1 2 Enterprise Systems Architecture/390 Common I/O-Device Commands, Second Edition, IBM, April 1992, SA22-7204-01
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