Execution of Input/Output Operations
The channel can execute six commands:
Write
Read
Read backward
Control
Sense
Transfer in channel
Each command except transfer in channel initiates
a correspondingI/O operation. The term "I/O oper ation" refers to the activity initiated by a command in
theI/O device and subchannel. The subchannel is in
volved with the execution of the operation from the
initiation of the command until the channel-end signal
is received or, in the case of command chaining, until
the device-end signal is received. The operation in the
device lasts until device end occurs.Blocking of Data
Data recorded on an external document may be di
vided into blocks. A block of data is defined for each
type of110 device as the amount of information rec
orded in the interval between adjacent starting and
stopping points of the device. The length of a block
depends on the documents; for example, a block can
be a card, a line of printing, or the information rec
orded between two consecutive gaps on tape.
The maximum amount of information that can be
transferred in oneI/O operation is one block. An I/O operation is terminated when the associated storage
area is exhausted or the end of the block is reached
whichever occurs first. For some operations, such writing on a magnetic tape unit or on an inquiry sta
tion, blocks are not defined, and the amount of in
formation transferred is controlled only by the pro
gram.Channel Address Word
The channel address word ( CAW) specifies the storage
protection key and the address of the first ccw associ
ated withSTART I/O. It appears at location 72. The
channel refers to the CAW only during the execution
ofSTART I/O. The pertinent information thereafter is
stored in the channel, and the program is free to
change the content of the CAW. Fetching of the CAW
by the channel does not affect the contents of location
72.
The CAW has the following format:I Key I 0 () 0 0 I Command Address
o 34 78 31
The fields in the CAW are allocated for the follow
ing purposes:
96Protection Key: Bits 0-3 form the storage protection
key for all commands associated withSTART I/O. This
key is matched with a storage key whenever data are
placed in storage.
Command Address: Bits 8-31 designate the location
of the first ccw in main storage.
Bit positions 4-7 of the CAW must contain zeros.
When the protection feature is not implemented, the
protection key must be zero. The three low-order bits
of the command address must be zero to specify the
ccw on integral boundaries for double words. If any
of these restrictions is violated or if the command ad
dress specifies a location outside the main storage of
the particular installation,START I/O causes the status
portion of the csw to be stored with the program
check bit on. In this event, theI/O operation is not
initiated.Channel Command Word
The channel command word (ccw) specifies the com
mand to be executed and, for commands initiatingI/O operations, it designates the storage area associ
ated with the operation and the action to be taken
whenever transfer to or from the area is completed.
The ccw' s be located anywhere in main storage,
and more than one can be associated with aSTART I/O. The channel refers to a ccw in main storage only
once, whereupon the pertinent information is stored
in the channel.
The first ccw is fetched during the execution ofSTART I/O. Each additional ccw in the chain is ob
tained when the operation has progressed to the point
where the additional ccw is needed. Fetching of the
CCw's by the channel does not affect the contents of
the location in main storage.
The ccw has the following format:I Command
.Code Data Address
o 78Flags Count 32 36 37 3940 4748 63
The fields in the ccw are allocated for the following
purposes:
Command Code: Bits0-7 specify the operation to
he performed.
Data Address: Bits 8-31 specify the location of an
eight-bit byte in main storage. It is the first location
referred to in the area designated by the ccw.
Chain-Data Flag: Bit 32, when one, specifies chain
ing of data. It causes the storage area designated by
the next ccw to be used with the current operation.
Whcn bit 32 is zero, the current control word is the
last one for the operation.
The channel can execute six commands:
Write
Read
Read backward
Control
Sense
Transfer in channel
Each command except transfer in channel initiates
a corresponding
the
volved with the execution of the operation from the
initiation of the command until the channel-end signal
is received or, in the case of command chaining, until
the device-end signal is received. The operation in the
device lasts until device end occurs.
Data recorded on an external document may be di
vided into blocks. A block of data is defined for each
type of
orded in the interval between adjacent starting and
stopping points of the device. The length of a block
depends on the documents; for example, a block can
be a card, a line of printing, or the information rec
orded between two consecutive gaps on tape.
The maximum amount of information that can be
transferred in one
area is exhausted or the end of the block is reached
whichever occurs first. For some operations, such
tion, blocks are not defined, and the amount of in
formation transferred is controlled only by the pro
gram.
The channel address word ( CAW) specifies the storage
protection key and the address of the first ccw associ
ated with
channel refers to the CAW only during the execution
of
stored in the channel, and the program is free to
change the content of the CAW. Fetching of the CAW
by the channel does not affect the contents of location
72.
The CAW has the following format:
o 34 78 31
The fields in the CAW are allocated for the follow
ing purposes:
96
key for all commands associated with
key is matched with a storage key whenever data are
placed in storage.
Command Address: Bits 8-31 designate the location
of the first ccw in main storage.
Bit positions 4-7 of the CAW must contain zeros.
When the protection feature is not implemented, the
protection key must be zero. The three low-order bits
of the command address must be zero to specify the
ccw on integral boundaries for double words. If any
of these restrictions is violated or if the command ad
dress specifies a location outside the main storage of
the particular installation,
portion of the csw to be stored with the program
check bit on. In this event, the
initiated.
The channel command word (ccw) specifies the com
mand to be executed and, for commands initiating
ated with the operation and the action to be taken
whenever transfer to or from the area is completed.
The ccw' s
and more than one can be associated with a
once, whereupon the pertinent information is stored
in the channel.
The first ccw is fetched during the execution of
tained when the operation has progressed to the point
where the additional ccw is needed. Fetching of the
CCw's by the channel does not affect the contents of
the location in main storage.
The ccw has the following format:
.
o 78
The fields in the ccw are allocated for the following
purposes:
Command Code: Bits
he performed.
Data Address: Bits 8-31 specify the location of an
eight-bit byte in main storage. It is the first location
referred to in the area designated by the ccw.
Chain-Data Flag: Bit 32, when one, specifies chain
ing of data. It causes the storage area designated by
the next ccw to be used with the current operation.
Whcn bit 32 is zero, the current control word is the
last one for the operation.