cation programs written for System/360 that satisfy
the conditions described in "Compatibility." Availability
Availability is the capability of a system to accept
and successfully process an individual job. System/370 machine facilities permit increased
availability by (1) allowing a larger number and a
broader range of jobs to be processed concurrently,
thus making the system more readily accessible to
any particular job, and (2) limiting the effect of an
error and identifying more precisely its cause, with
the result that the number of jobs affected by errors
is minimized and the correction of the errors is facili­
tated.
Several design aspects make this possible. A program is checked for the correctness of
instructions and data as the program is execut­
ed, and program errors are indicated separately
from equipment errors. Such checking and re­
porting assists in locating failures and isolating
effects. The protection facility, in conjunction with dy­
namic address translation, permits the protec­
tion of the contents of main storage from de­
struction or misuse caused by erroneous or
unauthorized storing or fetching by a program.
This provides increased security for the user,
thus permitting applications with different se­
curity requirements to be processed concur­
rently with other applications. Dynamic address translation allows isolation of
one application from another, still permitting
them to share common resources. Also, it per­
mits the implementation of virtual machines,
which may be used in the design and testing of
new versions of operating systems along with
the concurrent processing of application pro-
grams. Additionally, it provides for the concur­
rent operation of incompatible operating sys­
tems. Multiprocessing permits better use of storage
and processing capabilities, more efficient
communication between CPUs, and duplica­
tion of resources, thus aiding in the continua­
tion of system operation in the event of ma­
chine failures. Monitoring, program-event recording, and the
high-resolution timing facilities permit the test­
ing and debugging of programs without manual
intervention and with little effect on the con­
current processing of other programs. Emulation is performed under supervisory pro­
gram control, thus making it possible to per­
form emulation concurrently with other appli­
cations. On most models, error checking and correction
(ECC) in main storage, instruction retry, and
command retry provide for circumventing in­
termittent equipment malfunctions, thus reduc­
ing the number of equipment failures . An enhanced machine-check handling mecha­
nism provides model-independent fault isola­
tion, which reduces the number of programs
impacted by uncorrected errors. Additionally,
it provides model-independent recording of
machine-status information. This leads to
greater machine-check handling compatibility
between models and improves the capability
for loading and running a program on a differ­
ent model when a system failure occurs. A small number of manual controls are required
for basic system operation, permitting most
operator-system interaction to take place via a
unit operating as an I/O device and thus reduc­
ing the possibility of accidental operator errors.
IBM System/370 11
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