R
Field Reg. Number Control Registers r----- 32 General Registers r----- 32 Bits Floating-Point Registers I ........ ----64 Bits 0000 0 0001 0010 2 0011 3 0100 4 0101 5 0110 6 0111 7 1000 8 1001 9 1010 10 1011 11 1100 12 1101 13 1110 14
1111 15
Note: The braces indicate that the two registers may be coupled as a double-register
pair, designated by the R
field of the lower-numbered register. For example, the
general register pair 0 and 1 is designated by the R
field of register O. General, Floating-Point, and Control Registers
Input and Output
Input/output (I/O) operations involve the transfer
of information between main storage and an I/O device. I/O devices attach to channels, which con­
trol the transfer of data between the devices and
main storage.
Channels
The channel connects with the CPU and main stor­
age and, usually by means of the I/O interface, with
control units. The 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 may be an independent unit, complete
with necessary logical and storage capabilities, or it
may time-share CPU facilities and be physically
integrated with the CPU. In either case, channel
functions are identical. Channels may be implement­
ed, however, to have different maximum data­
transfer capabilities. System/370 has three types of channels: byte­
multiplexer, block-multiplexer, and selector chan­
nels.
Input/Output Interface
For most devices, communication between the con­
trol unit and the channel takes place over a connec­
tion called the I/O interface. The I/O interface System Organization 17

provides an information format and control signal
sequences that are independent of the type of con­
trol unit and channel and provide a uniform means
of attaching and controlling various types of I/O devices. I/O devices that do not use the I/O interface
employ the same information format and control
signal sequences. Devices and Control Units
Input/ output devices include such equipment as card
readers and punches, magnetic tape units, disk stor­
age, drum storage, typewriter-keyboard devices,
printers, teleprocessing devices, and sensor-based
equipment.
Many I/O devices function with an external doc­
ument, such as a punched card or a reel of magnetic
tape. Some I/O devices handle only electrical sig­
nals, such as those found in sensor-based networks.
In either case, I/O device operation is regulated by a
control unit. The control-unit function may be
housed with the I/O device or in the CPU, or a sep-
18 System/370 Principles of Operation
arate control unit may be used. In all cases, the
control-unit function provides the logical and buffer­
ing 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.
System Console
The system console provides the functions necessary
to operate and control the system. It consists of a
system control panel and, in most cases, an associat­
ed console device, which may also be used as an I/O device for communicating with the supervisory pro­
gram and problem programs. The need for operator
manipulation of manual controls is held to a mini­
mum by the system design and the governing super­
visory program.
The main functions provided by the system con­
sole include power-on/ off, reset, initial-pro gram­
loading, start/stop, and display and enter functions.