Input jOutput Input / Output Devices and Control Units
Input/output operations involve the transfer of infor
mation to or from main storage and anI/O device.
Input/output devices include such equipment as card
read punches, magnetic tape units, disk storage, drum
storage, typewriter-keyboard devices, printers, TELEPROCESSINC@ devices, and process control equipment.
ManyI/O devices function with an external docu
ment, such as a punched card or a reel of magnetic
tape. SomeI/O devices handle only electrical signals,
such as those found in process-control networks. In
either case,I/O device operation is regulated by a
control unit. The control-unit function may be housed
with theI/O device, as is the case with a printer, or a
separate control unit may be used. In all cases, the
control-unit function provides the logical andbuffer ing capabilities necessary to operate the associated I/O device. From the programming point of view,
most control-unit functions merge withI/O device
functions.
Each control unit functions only with theI/O de
vice for which it is designed, but each control unit
has standard-signal connections with regard to the
channel to which it is attached.Input / Output Interface So that the CPU may control a wide variety of I/O devices, all control units are designed to respond to a
standard set of signals from the channel. This control
unit-to-channel connection is called theI/O interface.
It enables theCPU to handle all I/O operations with
only four instructions.Channels Channels connect with the CPU and main storage and,
via theI/O interface, with control units. Each chan
nel has facilities for:
AcceptingI/O instructions from the CPU Addressing devices specified by I/O instructions
Fetching channel control information from main storage
Decoding control information
Testing information for validity
Executing control information
Providing control signals to theI/O interface
Accepting control-response signals from theI/O interface
Buffering data transfersChecking parity of bytes transferred Counting the number of bytes transferred
Accepting status information fromI/O devices
Maintaining channel-status information
Sending requested status information to main storage
Sequencing interruption requests fromI/O devices
Signaling interruptions to theCPU A channel may be an independent unit, complete
with necessary logical and storage capabilities, or it
may shareCPU facilities and be physically integrated
with theCPU. In either case, channel functions are
identical.
The System/360 has two types of channels: multi
plexor and selector. The channel facility necessary to
sustain an operation with anI/O device is called a
subchannel. The selector channel has one subchannel'
the multiplexor channel has multiple subchannels. '
Channels have two modes of operation: burst and
multiplex.
In the burst mode, all channel facilities are mono
polized for the duration of data transfer to or from a
particularI/O device. The selector channel functions
only in the burst mode.
The multiplexor channel functions in both the burst
mode and in the multiplex mode. In the latter mode,
the multiplexor channel sustains simultaneousI/O op
erations on several subchannels. Bytes of data are
interleaved together and then routed to or from the
selectedI/O devices and to or from the desired loca
tions in main storage.Input / Output Instructions The System/360 uses only four I/O instructions: START I/O TEST CHANNEL TEST I/O HALT I/O Input/output instructions can be executed only
while theCPU is in the supervisor state. Start I/O The START I/O initiates an I/O operation. The adoress part of the instruction specifies the channel and I/O device.
Test Channel
TheTEST CHANNEL scts the condition code in the psw
to indicate the state of the channel addressed bvthe instruction. The condition code then indicates nel available, interruption condition in channel, chan
neJ working, or channel not operational.
TestI/O The TEST I/O causes a csw to be stored in location 64
of main storage, if the device addressed byTEST I/O has specified conditions for interruption. The csw
provides information on the status of the channel andI/O devices.
HaltI/O The HALT I/O terminates a channel operation.
System Structure 19
Input/output operations involve the transfer of infor
mation to or from main storage and an
Input/output devices include such equipment as card
read punches, magnetic tape units, disk storage, drum
storage, typewriter-keyboard devices, printers, TELE
Many
ment, such as a punched card or a reel of magnetic
tape. Some
such as those found in process-control networks. In
either case,
control unit. The control-unit function may be housed
with the
separate control unit may be used. In all cases, the
control-unit function provides the logical and
most control-unit functions merge with
functions.
Each control unit functions only with the
vice for which it is designed, but each control unit
has standard-signal connections with regard to the
channel to which it is attached.
standard set of signals from the channel. This control
unit-to-channel connection is called the
It enables the
only four instructions.
via the
nel has facilities for:
Accepting
Fetching channel control information from main storage
Decoding control information
Testing
Executing control information
Providing control signals to the
Accepting control-response signals from the
Buffering data transfers
Accepting status information from
Maintaining channel-status information
Sending requested status information to main storage
Sequencing interruption requests from
Signaling interruptions to the
with necessary logical and storage capabilities, or it
may share
with the
identical.
The System/360 has two types of channels: multi
plexor and selector. The channel facility necessary to
sustain an operation with an
subchannel. The selector channel has one subchannel'
the multiplexor channel has multiple subchannels. '
Channels have two modes of operation: burst and
multiplex.
In the burst mode, all channel facilities are mono
polized for the duration of data transfer to or from a
particular
only in the burst mode.
The multiplexor channel functions in both the burst
mode and in the multiplex mode. In the latter mode,
the multiplexor channel sustains simultaneous
erations on several subchannels. Bytes of data are
interleaved together and then routed to or from the
selected
tions in main storage.
while the
Test Channel
The
to indicate the state of the channel addressed bv
neJ working, or channel not operational.
Test
of main storage, if the device addressed by
provides information on the status of the channel and
Halt
System Structure 19