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IBM System/370
Extended Architecture
Principles of Operation
Publication Number File Number
SA22-7085-1 S370-01
Second Edition (January 1987)
This major revision obsoletes SA22-7085-0 and Technical Newslet-
ters SN22-0682 and SN22-0688.
For a summary of changes to the last edition, see the last
section of the Preface.
Except for minor style alterations, changes are identified by a
vertical line to the left of the change.
Changes are made periodically to the information herein; before
using this pUblication in connection with the operation of IBM
equipment, refer to the latest IBM System/370, 30xx, and 4300
Processors Bibliography, GC20-0001, for the editions that are
applicable and current.
References in this publication to IBM products, programs, or
services do not imply that IBM intends to make these available in
all countries in which IBM operates. Any reference to an IBM
program product in this publication is not intended to state or
imply that only IBM's program product may be used. Any func-
tionally equivalent program may be used instead.
Publications are not stocked at the address given below.
Requests for IBM publications should be made to your IBM repre-
sentative or to the IBM branch office serving your locality.
A form for readers' comments is provided at the back of this
publication. If the form has been removed, comments may be
addressed to IBM Corporation, Central Systems Architecture,
Department E57, PO Box 390, Poughkeepsie, NY, USA 12602. IBM may
use or distribute whatever information you supply in any way it
believes appropriate without incurring any obligation to you.
ec) Copyright International Business Machines Corporation 1983,
1987
PREFACE
This publication provides, for reference and phrases appear, of necessity, earli-
purposes, a detailed definition of the er in the publication than the principal
machine functions performed by systems discussions explaining them. The reader
operating in the System/370 extended- who encounters a problem because of this
architecture (370-XA) mode. arrangement should refer to the index,
which indicates the location of the key
The publication applies only to systems description.
operating in the 370-XA mode. The IBM
System/370 Principles of Operation, The information presented in this publi-
GA22-7000, should be consulted regarding cation is grouped in 17 chapters and
the functions of the architecture which several appendixes:
apply to systems operating in the
System/370 mode. Chapter 1, Introduction, highlights some
of the major facilities of systems oper-
The publication describes each function ating in the 370-XA mode.
at the level of detail needed to prepare
an assembler-language program that Chapter~, Organization, describes the
relies on that function. It does not, major groupings within the system -- the
however, describe the notation and central processing unit (CPU), storage,
conventions that must be employed in and input/output -- with some attention
preparing such a program, for which the given to the composition and character-
user must instead refer to the appropri- istics of those groupings.
ate assembler-language publication.
Chapter d, Storage, explains the infor-
The information in this publication is mation formats, the addressing of stor-
provided principally for use by age, and the facilities for storage
assembler-language programmers, although protection. It also deals with dynamic
anyone concerned with the functional address translation (OAT), which,
details of systems operating in the coupled with special programming
370-XA mode will find it useful. support, makes the use of a virtual
storage possible in systems operating in
This publication is written as a refer- the 370-XA mode. Dynamic address trans-
ence and should not be considered an lation eliminates the need to assign a
introduction or a textbook. It assumes program to a fixed location in real
the user has a basic knowledge of data- storage and thus reduces the addressing
processing systems. IBM publications constraints on system and problem
relating to systems operating in the programs.
370-XA mode are listed and described in
the IBM System/370, 30xx, and 4300 Chapter !, Control, describes the facil-
Processors Bibliography, GC20-0001. ities for the switching of system
status, for special externally initiated
All facilities discussed in this publi- operations, for debugging, and for
cation are not necessarily available on timing. It deals specifically with CPU
every model. Furthermore, in some states, control modes, the program-
instances the definitions have been status word (PSW), control registers,
structured to allow for some degree of program-event recording, timing facili-
extendibility, and therefore certain ties, resets, store status, and initial
capabilities may be described or implied program loading.
that are not offered on any model.
Examples of such capabilities are the Chapter 2, Program Execution, explains
use of a 16-bit field in the sUbsystem- the role of instructions in program
identification word to identify the execution, looks in detail at instruc-
channel subsystem, the size of the CPU tion formats, and describes briefly the
address, and the number of CPUs sharing use of the program-status word (PSW), of
main storage. The allowance for this branching, and of interruptions. It
type of extendibility should not be also details the aspects of program
construed as implying any intention by execution on one CPU as observed by
IBM to provide such capabilities. For other CPUs and by channel programs.
information about the characteristics
and availability of facilities on a Chapter ~,Interruptions, details the
specific model, see the functional char- mechanism that permits the CPU to change
acteristics publication for that model. its state as a result of conditions
The availability of facilities is summa- external to the system, within the
rized in the IBM ~YEtemL370 System system, or within the CPU itself. Six
Summary: Processors, GA22-7001. classes of interruptions are identified
and described: machine-check interrup-
largely because this publ i cat ion is tions, program interruptions, super-
arranged for re'ference, certain words visor-call interruptions, external
iii
1 nterrupt 1 ons, input/output 1 nterrup-