Control General Floating-Point Registers Registers Registers R Register Field Number 1+--32 1+--32 1 64 bits 0000 0 [I I 0001 1 0010 2 [I 0011 3 I [I 0100 4 I 0101 5 [I 0110 6 I 0111 7 1000 8 [I 1001 9 I [I Note: The brackets 1010 10 indicate that the two registers may be coupled as a double-register I pair, designated by 1011 11 specifying the lower-
numbered register in the R field. For ex- [I ample, the general- 1100 12 register pair 14 and
15 is designated by 1110 binary in the R I field. 1101 13 [I 1110 14 I 1111 15 General, Floating-Point, and Control Registers Chapter 2. Organization 2-5

CHANNEL SETS The group of channels which connects to
a particular CPU is called a channel
set. When channel-set switching is
installed in a multiprocessing config­
uration, the program can control which CPU is connected to a particular channel
set. A CPU can be connected to no more
than one channel set at a time, and a
channel set can be connected to no more
than one CPU at a time. When channel­
set switching is not installed, the channel sets, in the absence of model­
dependent reconfiguration controls, are permanently connected to a single CPU. CHANNELS A channel relieves the CPU of the burden
of communicating directly with I/O devices and permits data processing to
proceed concurrently with I/O operations. A channel is connected with
main storage, with control units, and, unless it is a member of a disconnected
channel set, with a cpu.
A channel may be an independent unit,
complete with the necessary logical and
internal-storage capabilities, or it may
time-share CPU facilities and be phys­
ically integrated with the CPU. In
either case, the functions performed by a channel are identical. The maximum
data-transfer rate may differ, however,
depending on the implementation.
There are three types of channels:
byte-multiplexer, block-multiplexer, and
selector channels.
2-6 System/370 Principles of Operation I/O DEVICES AND CONTROL UNITS I/O devices include such equipment as
card readers and punches, magnetic-tape
units, direct-access storage, displays,
keyboards, printers, teleprocessing
devices, communications controllers, and
sensor-based equipment. Many I/O devices function with an external
medium, such as punched cards or magnet­
ic tape. Some I/O devices handle only
electrical signals, such as those found
in sensor-based networks. In either
case, I/O-device operation is regulated
by a control unit. In all cases, the
control-unit function provides the
logical and buffering capabilities
necessary to operate the associated I/O device. From the programming point of
view, most control-unit functions merge
with I/O-device functions. The
control-unit function may be housed with
the I/O device or in the CPU, or a sepa­
rate control unit may be used. OPERATOR FACILITIES The operator facilities provide the
functions necessary for operator control
of the machine. Associated with the
operator facilities may be an operator­
console device, which may also be used
as an I/O device for communicating with
the program.
The main functions provided by the oper­
ator facilities include resetting,
clearing, initial program loading,
start, stop, alter, and display.