CHAPTER ORGANIZATION Main Storage •.••.••••••.••••••.•.••••••..•••••.••••••••••. 2-3 CPU ..•••.••••••••••.••.•..••••••••..•••..••••.•.••...•.••• 2-3
PSW ••....•..••..•..••.•••••..•••••.••..••.••.•.•••.••... 2-3
General Registers •.••.•...••••..•••....•.....•......•.•. 2-4 Floating-Point Registers ••.•••.•..••••.••..••..•.••....• 2-4
Control Registers •..••••••.•••.••......•.•.•••......•••• 2-4
Vector Facility ..••.••..••••••••..••...•••.•••••.•.••.•• 2-4 1/0 .................•............•...........•.....•.•.... 2-4
Channel Sets .•..••.••......•.•......•...••••.•..••.•.••. 2-6
Channel s ..•....••••••.•.•....•...•••...••.••••..•••••.•. 2-6 1/0 Devices and Control Units ...•.•.•..•.•••..••••••.••. 2-6 Operator Facilities ........•...•.••......•..•....••..•.... 2-6
logically, System/370 consists of main
storage, one or more central processing
units (CPUs), operator facilities, chan­
nel sets, channels, and 1/0 devices. 1/0 devices are attached to channels
through control units. The physical
identity of these functions may vary
among implementations, called "models." The figure "logical Structure" depicts
the logical structure for a single-CPU system and for a two-CPU multiprocessing
system.
Specific processors may differ in their
internal characteristics, the installed
facilities, the number and types of
channels, the size of main storage, and the representation of the operator
facilities. The differences in internal
characteristics are apparent to the
observer only as differences in machine
performance.
Chapter 2. Organization 2-1

Single-CPU Configuration Main
Storage CPU /
/ 2-CPU Multiprocessing Configuration Main /
Storage I CPU CPU / Channel Channel Channel Channel Channel Channel Channel Set Channel Set Channel Set 1/ /1 1 1 1 1 0 0 1 1 1 1 1 1/ /1 1 1 1 1 1 1 1 1 1 1 1/ o 0 0 0 0 0 o 0 0 0 0 0 o 0 0 0 0 0 logical Structure
A system viewed without regard to its
I/O devices ;s referred to as a config­
uration. All of the physical equipment,
whether in the configuration or not, is
referred to as the installation.
Model-dependent reconfiguration controls
may be provided to change the amount of main storage and the number of CPUs, channels, and channel sets ;n the
configuration. In some instances, the
reconfiguration controls may be used to
partition a single configuration into
multiple configurations. Each of the
configurations so reconfigured has the same structure, that is, main storage,
one or more CPUs, and channels. Each
configuration is isolated in that the main storage in one configuration is not
2-2 System/370 Principles of Operation directly addressable by the CPUs and
channels in another configuration. It is, however, possible for one configura­
tion to communicate with another by
means of shared I/O devices, the direct-control facility, or a channel­
to-channel adapter. At anyone time, the storage, CPUs, channel sets, and
channels connected together in a system
are referred to as being in the config­
uration. Channel sets and the
associated channels are considered to be
in a particular configuration as long as
they are connected to main storage inde­
pendent of whether or not the channel
set is connected to a CPU in the config­
uration. Each CPU, channel set,