vided by an interruption during execution of the op
eration.
Program-controlled interruption provides a means
of alerting the program of the progress of chaining
during anI/O operation. It permits programmed dy
namic main-storage allocation.Commands The following table lists the command codes for the
six commands and indicates which Hags are defined
for each command. The Hags are ignored for all com
mands for which they are not defined.
Write
Read
NAME
Read backwardControl Sense
Transfer in channelNOTES FLAG CD CC SLI PCI CD CC SLI SKIP PCI CD CC SLI SKIP PCI CD CC SLI PCI CD CC SLI SKIP PCI CD Chain data CC Chain command SLI Suppress incorrect length SKIP Skip PCI Program-controlled interruption CODE MMMMMMOI MMMMMM10 MMMM 1100 MMMMMMII
MMMM0100 x x x x 1000 All Hags have individual significance, except that
the cc andSLI Hags are ignored when the CD Hag is on.
TheSLI Hag is ignored on immediate operations, in
which case the incorrect-length indication is sup
pressed regardless of the setting of the Hag. ThePCI Hag is ignored during initial program loading.
Write
A write operation is initiated at theI/O device, and the
subchannel is set up to transfer data from main storage
to theI/O device. Data in storage are fetched in an
ascending order of addresses, starting with the ad
dress specified in the ccw.
A ccw used in a write operation is inspected for theCD, CC, SLI, and the PCI Hags. The setting of the skip
Hag is ignored. Bit positions0-5 of the ccw contain
modifier bits.
Programming NoteOn writing magnetic tape, block-length is not defined,
and the amount of data written is controlled only by
the count in the ccw. Every operation terminated
under count control causes the incorrect-length indica
tion, unless the indication is suppressed by theSLI Hag.
Read
A read operation is initiated at theI/O device, and the
subchannel is set up to transfer data from the device
to main storage. For devices such as magnetic tape
units, disk storage, and card equipment, the bytes of
data within a block are provided in the same sequence
as written by means of a write command. Data in
storage are placed in an ascending order of addresses,
starting with the address specified in the ccw.
A ccw used in a read operation is inspected for
every·one of the five Hags -CD, CC, SLI, skip, and PCI. Bit positions 0-5 of the ccw contain modifier bits.
Read Backward
A read-backward operation is initiated at theI/O de
vice, and the subchannel is set up to transfer data
from the device to main storage.On magnetic tape
units, read backward causes reading to be performed
with the tape moving backwards. The bytes of data
within a block are sent to the channel in a sequence
opposite to that on writing. The channel places the
bytes in storage in a descending order of address, start
ing with the address specified in the ccw. The bits
within an eight-bit byte are in the same order as sent
to the device on writing.
A ccw used in a read-backward operation is in
spected for everyone of the five Hags -CD, cc, SLI, skip, and PCI. Bit positions 0-3 of the ccw contain
modifier bits.
Programming Note
When data chaining is used during a read-backward
operation, the channel places data in storage in a de
scending sequence but fetches ccw's in an ascending
sequence. Consequently, if a magnetic tape is to be
written so that it can be read in either the forward or
backward direction as a self-describing record, the
ccw must be written at both the beginning and the
end of the physical record. If more than one ccw is to
be used, the order of the ccw's must be reversed at the
end of the record since the storage areas associated
with the ccw's are used in reverse sequence. Further
more, a ccw used for reading backward must describe
the associated storage area by specifying the highest
address of the area, whereas it normally cont,IS the
lowest address.
Control
A control operation is initiated at theI/O device, and
the subchannel is set up to transfer data from main
storage to the device. The device interprets the data
as control information. The control information, if
any, is fetched from storage in an ascending order of
addresses, starting with the address specified in the
ccw. A control command is used to initiate at theI/O device an operation not involving transfer of data -
such as backspacing or rewinding magnetic tape or
positioning a disk access mechanism.
For most control functions, the entire operation is
specified by the modifier bits in the command code,Input/Output Operations 101
eration.
Program-controlled interruption provides a means
of alerting the program of the progress of chaining
during an
namic main-storage allocation.
six commands and indicates which Hags are defined
for each command. The Hags are ignored for all com
mands for which they are not defined.
Write
Read
NAME
Read backward
Transfer in channel
MMMM
the cc and
The
which case the incorrect-length indication is sup
pressed regardless of the setting of the Hag. The
Write
A write operation is initiated at the
subchannel is set up to transfer data from main storage
to the
ascending order of addresses, starting with the ad
dress specified in the ccw.
A ccw used in a write operation is inspected for the
Hag is ignored. Bit positions
modifier bits.
Programming Note
and the amount of data written is controlled only by
the count in the ccw. Every operation terminated
under count control causes the incorrect-length indica
tion, unless the indication is suppressed by the
Read
A read operation is initiated at the
subchannel is set up to transfer data from the device
to main storage. For devices such as magnetic tape
units, disk storage, and card equipment, the bytes of
data within a block are provided in the same sequence
as written by means of a write command. Data in
storage are placed in an ascending order of addresses,
starting with the address specified in the ccw.
A ccw used in a read operation is inspected for
every
Read Backward
A read-backward operation is initiated at the
vice, and the subchannel is set up to transfer data
from the device to main storage.
units, read backward causes reading to be performed
with the tape moving backwards. The bytes of data
within a block are sent to the channel in a sequence
opposite to that on writing. The channel places the
bytes in storage in a descending order of address, start
ing with the address specified in the ccw. The bits
within an eight-bit byte are in the same order as sent
to the device on writing.
A ccw used in a read-backward operation is in
spected for everyone of the five Hags -
modifier bits.
Programming Note
When data chaining is used during a read-backward
operation, the channel places data in storage in a de
scending sequence but fetches ccw's in an ascending
sequence. Consequently, if a magnetic tape is to be
written so that it can be read in either the forward or
backward direction as a self-describing record, the
ccw must be written at both the beginning and the
end of the physical record. If more than one ccw is to
be used, the order of the ccw's must be reversed at the
end of the record since the storage areas associated
with the ccw's are used in reverse sequence. Further
more, a ccw used for reading backward must describe
the associated storage area by specifying the highest
address of the area, whereas it normally cont
lowest address.
Control
A control operation is initiated at the
the subchannel is set up to transfer data from main
storage to the device. The device interprets the data
as control information. The control information, if
any, is fetched from storage in an ascending order of
addresses, starting with the address specified in the
ccw. A control command is used to initiate at the
such as backspacing or rewinding magnetic tape or
positioning a disk access mechanism.
For most control functions, the entire operation is
specified by the modifier bits in the command code,