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IBM System/370




Principles of Operation




Publication Number File Number
GA22-7000-10 S370-01
Eleventh Edition (September 1987)

This major revision obsoletes and replaces GA22-7000-8, GA22-
7000-9, and Technical Newsletters GN22-0644 and GN22-0683.
Significant changes or additions to the text and illustrations
are indicated by a vertical line to the left of the change.
Changes are made occasionally 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, GC20-000I7 for the editions that are
applicable and current.
IBM may have patents or pending patent applications covering
subject matter described herein. Furnishing this publication
does not constitute or imply a grant of any license under any
patents, patent applications, trademarks, copyrights, or other
rights of IBM or of any third party, or any right to refer to IBM
in any advertising or other promotional or marketing activities.
IBM assumes no responsibility for any infringement of patents or
other rights that may result from the use of this publication or
from the manufacture, use, lease, or sale of apparatus described
herein.
licenses under IBM's utility patents are available on reasonable
and nondiscriminatory terms and conditions. Inquiries relative
to licensing should be directed, in writing, to: IBM Corpora-
tion, Director of Contracts and licensing, Armonk, NY, USA 10504.
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.
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 reader's 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 1970,
1972, 1973, 1974, 1980, 1981, 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 discussions explaining them. The reader
System/370. who encounters a problem because of this
arrangement should refer to the index,
The publication applies only to systems which indicates the location of the key
operating in the System/370 mode. The description.
IBM 370-XA Principles of Operation,
SA22-7085, should be consulted regarding The information presented in this publi-
the functions of the architecture which cation is grouped in 13 chapters and
apply to systems operating in the 370-XA several appendixes:
mode, and the IBM 4300 Processors Prin-
ciples of Operation for ECPS:VSE Mode, Chapter 1, Introduction, highlights some
GA22-7070, should be consulted regarding of the major facilities of System/370.
the functions of the architecture which
apply only to systems operating in the Chapter~, Organization, describes the
VSE mode. major groupings within the system -- the
central processing unit (CPU), storage,
The publication describes each function and input/output -- with some attention
at the level of detail needed to prepare given to the composition and character-
an assembler-language program that istics of those groupings.
relies on that function. It does not,
however, describe the notation and Chapter d, Storage, explains the infor-
conventions that must be employed in mation formats, the addressing of stor-
preparing such a program, for which the age, and the facilities for storage
user must instead refer to the appropri- protection. It also deals with dynamic
ate assembler-language publication. address translation (OAT), which,
coupled with special programming
The information in this publication is support, makes the use of a virtual
provided principally for use by storage possible in System/370. Dynamic
assembler-language programmers, although address translation eliminates the need
anyone concerned with the functional to assign a program to a fixed lo~ation
details of System/370 will find it in real storage and thus reduces the
useful. addressing constraints on system and
problem programs.
This publication is written as a refer-
ence and should not be considered an Chapter ~, Control, describes the facil-
introduction or a textbook. It assumes ities for the switching of system
the user has a basic knowledge of data- status, for special externally initiated
processing systems. IBM publications operations, for debugging, and for
relating to System/370 are listed and timing. It deals specifically with CPU
described in the IBM System/370, 30xx, states, control modes, the program-
and 4300 Processors Bibliography, status word (PSW), control registers,
GC20-0001. program-event recording, timing facili-
ties, resets, store status, and initial
All facilities discussed in this publi- program loading.
cation are not necessarily available on
every model. Furthermore, in some Chapter 2, Program Execution, explains
instances the definitions have been the role of instructions in program
structured to allow for some degree of execution, looks in detail at instruc-
extendibility, and therefore certain tion formats, and describes briefly the
capabilities may be described or implied use of the program-status word (PSW), of
that are not offered on any model. branching, and of interruptions. It
Examples of such capabilities are the contains the principal description of
number of channel-mask bits in the the dual-address-space (DAS) facility.
control register, the size of the CPU It also details the aspects of program
address, and the number of CPUs sharing execution on one CPU as observed by
main storage. The allowance for this other CPUs and by channels.
type of extendibility should not be
construed as implying any intention by Chapter ~, Interruptions, details the
IBM to provide such capabilities. For mechanism that permits the CPU to change
information about the characteristics its state as a result of conditions
and availability of facilities on a external to the system, within the
specific model, see the functional char- system, or within the CPU itself. Six
acteristics publication for that model. classes of interruptions are identified
and described: machine-check interrup-
~ Largely because this publication is tions, program interruptions, super-
~ arranged for reference, certain words visor-call interruptions, external

iii
interruptions, input/output interrup-