Types of Channels
A system can be equipped with two types of channels:
selector and multiplexor.Channels are classified ac
cording to the modes of operation they can sustain.
The channel facilities required for sustaining a
singleI/O operation are termed a subchannel. The
subchannel consists of the channel storage used for
recording the addresses, count, and any status and
control information associated with theI/O operation.
The mode in which a channel can operate depends
upon whether it has one or more subchannels.
The selector channel has only one subchannel and
operates only in the burst mode. The burst always
extend over the whole block of data, or, when com
mand chaining is specified, over the whole sequence
of blocks. The selector channel cannot perform any
multiplexing and therefore can be involved in only
one data transfer operation at a time. In the meantime,
otherI/O devices attached to the channel can execute
operations not involving communication with the
channel. When the selector channel is not executing
an operation or a chain of operations and is not proc
essing an interruption, it scans the attached devices for
status information.
The multiplexor channel contains multiple subchan
nels and can operate in either multiplex or burst mode.
It can switch between the two modes at any time, and
an operation on anyone sub channel can occur partial
ly in the multiplex and partially in the burst mode.
When the multiplexor channel operates in multiplex
mode, it can sustain concurrently one J/0 operation
per subchannel, provided that the total load on the
channel does not exceed itscapacity. 'To the program, each subchannel as an independent selector
channel.''''hen the multiplexor channel is not servic ing an I/O device, it scans its devices for data and for
interruption conditions.''''hen thc rnu]tiplexo1' channel operat('s in burst modC', tlw suhchannel associated vilith the ImI'st opera
tionmonopolizes all channel htc'ilitics and appears to the as a single selector channel. The remaining 5U hchanncls on the multiplexor chan
nel must remain dormant and cannot respond to de
vices until the burst is completed.System Operation
Input/output operations arc initiated and controlled
by information withtlUCt; typcs of formats: instruc
tions, commands, and orders. Instructionsare decoded
and executed by theCPU and arc part of the CPU pro
gram.Commands are decoded and executed by the
channels, and initiateI/O operations, such as reading
and writing. Both instructions and commands are
fetched from main storage and are common to all
types ofI/O devices.
Functions peculiar to a device, such as rewinding
tape or spacing a line on the printer, are specified by
orders.Orders are decoded and executed by I/O de
vices. The execution of orders is initiated by a control
command, and the associated control information is
transferred to the device as data during the control
operation or is specified in the modifier bits of the
command code.
TheCPU program initiates I/O operations with the
instructionSTART I/O. This instruction identifies the
device and causes the channel to fetch the channel
address word (CAW) from a fixed location in main
storage. TheCAW contains the protection key and
designates the location in main storage from which
the channel subsequently fetches the first channel
command word (ccw). The ccw specifies the com
mand to be executed and the storage area, if any, to
be used.
If the channel is not operating in burst mode and if
the sub channel associated with the addressI/O device
is not busy, the channel attempts to select the device
by sending the address of the device to all attached
control units. A control unit that recognizes the ad
dress connects itself logically to the channel and re
sponds to the selection by returning the address. The
channel subsequently sends the command code over
the interface, and the device responds with a status
byte indicating whether it can execute the command.
At this time, the execution ofSTART I/O is termi
nated. The results of the attempt to initiate the execu
tion of the command are indicated by setting the
condition code in the program status word ( psw) ,
and, under certain conditions, by storing a portion of
the channel status word (csw).
If the operation is initiated at the device and its
execution involves transfer of data, the subchannel
is set up to respond to service requests from the device
and assumes further control of the operation. In the
case of operations that do not require any data to be
transferred to or from the device, the device may
signal the end of the operation immediately on receipt
of the command code.
AnI/O operation may involve transfer of data to
one storage area, designated by a single ccw, or, when
data chaining is specified, to a number of noncon
tiguous storage areas. In the latter case, a chain of
ccw's is used, in which each ccw designates an area
in main storage for the original operation. The pro
gram can be notified of the progress of chaining by
specifying that the channel interrupt the program
upon fetching a new ccw.Input/Output Operations 85
A system can be equipped with two types of channels:
selector and multiplexor.
cording to the modes of operation they can sustain.
The channel facilities required for sustaining a
single
subchannel consists of the channel storage used for
recording the addresses, count, and any status and
control information associated with the
The mode in which a channel can operate depends
upon whether it has one or more subchannels.
The selector channel has only one subchannel and
operates only in the burst mode. The burst always
extend over the whole block of data, or, when com
mand chaining is specified, over the whole sequence
of blocks. The selector channel cannot perform any
multiplexing and therefore can be involved in only
one data transfer operation at a time. In the meantime,
other
operations not involving communication with the
channel. When the selector channel is not executing
an operation or a chain of operations and is not proc
essing an interruption, it scans the attached devices for
status information.
The multiplexor channel contains multiple subchan
nels and can operate in either multiplex or burst mode.
It can switch between the two modes at any time, and
an operation on anyone sub channel can occur partial
ly in the multiplex and partially in the burst mode.
When the multiplexor channel operates in multiplex
mode, it can sustain concurrently one J
per subchannel, provided that the total load on the
channel does not exceed its
channel.
interruption conditions.
tion
nel must remain dormant and cannot respond to de
vices until the burst is completed.
Input/output operations arc initiated and controlled
by information with
tions, commands, and orders. Instructions
and executed by the
gram.
channels, and initiate
and writing. Both instructions and commands are
fetched from main storage and are common to all
types of
Functions peculiar to a device, such as rewinding
tape or spacing a line on the printer, are specified by
orders.
vices. The execution of orders is initiated by a control
command, and the associated control information is
transferred to the device as data during the control
operation or is specified in the modifier bits of the
command code.
The
instruction
device and causes the channel to fetch the channel
address word (
storage. The
designates the location in main storage from which
the channel subsequently fetches the first channel
command word (ccw). The ccw specifies the com
mand to be executed and the storage area, if any, to
be used.
If the channel is not operating in burst mode and if
the sub channel associated with the address
is not busy, the channel attempts to select the device
by sending the address of the device to all attached
control units. A control unit that recognizes the ad
dress connects itself logically to the channel and re
sponds to the selection by returning the address. The
channel subsequently sends the command code over
the interface, and the device responds with a status
byte indicating whether it can execute the command.
At this time, the execution of
nated. The results of the attempt to initiate the execu
tion of the command are indicated by setting the
condition code in the program status word ( psw) ,
and, under certain conditions, by storing a portion of
the channel status word (csw).
If the operation is initiated at the device and its
execution involves transfer of data, the subchannel
is set up to respond to service requests from the device
and assumes further control of the operation. In the
case of operations that do not require any data to be
transferred to or from the device, the device may
signal the end of the operation immediately on receipt
of the command code.
An
one storage area, designated by a single ccw, or, when
data chaining is specified, to a number of noncon
tiguous storage areas. In the latter case, a chain of
ccw's is used, in which each ccw designates an area
in main storage for the original operation. The pro
gram can be notified of the progress of chaining by
specifying that the channel interrupt the program
upon fetching a new ccw.