Chain-Command Flag: Bit 33, when one and when
the chain-data flag is off, specifics chaining of com
mands. It causes the operation specified by the com
mand code in the next ccw to be initiated on normal
completion of the current operation. Whcn bit 33 is
zero or when the CD flag is one, the next ccw docs not
specify a new command.
Suppress-Length-Indication Flag: Bit 34 controls
whether an incorrect length condition is to be indi
cated to the program. When this bit is one and the CD
flag is off in the last ccw used, the incorrect-length in
dication is suppressed. If the ccw has the cc flag on,
command chaining takes place. Absence of theSLI flag or the presence of the CD flag causes the program to
be notified of the incorrect-length condition when it
occurs.Skip Flag: Bit 35, when one, specifies suppression
of transfer of information to storage during a read,
read-backward, or sense operation. When bit 35 is
zero, normal transfer of data takes place.
Program-Controlled-Interruption Flag: Bit 36,
when one, causes the channel to generate an inter
ruption condition upon fetching the ccw. When bit 36
is zero, normal operation takes place.
Count: Bits 48-63 specify the number of eight-bit
byte locations in the storage area designated by the
ccw.
Bit positions 37-39 of every ccw other than one spe
cifying transfer in channel must contain zeros.Viola tion of this restriction generates the program-check
condition. When the first ccw designated by the CAW
does not contain the required zeros, thcI/O operation
is not initiated, and the status portion of the csw with
the program-check indication is stored during execu
tion ofSTART I/O. Detection of this condition during
data chaining causes theI/O device to be signaled to
terminate the operation. When the absence of these
zeros is detected during command chaining, the new
operation is not initiated, and no interruption condition
is generated.
The content of bit positions40-47 of the ccw is
ignored.Command Code The command code in the ccw specifies to the chan
nel and theI/O device the operation to be performed.
The two low-order bits or, when these bits are00, the four low-order bits of the command code iden
tify the operation to the channel. The channel dis
tinguishes among the following four operations:Output forward (write, control)
Input forward (read, sense)
Input backward (read backward)
Branching (transfer in channel)
The channel ignores the high-order bits of the com
mand code.
Commands that initiateI/O operations (write, read,
read backward, control, and sense) cause all eight bits
of the command code to be transferred to theI/O de
vice. In these command codcs, the high-order bit po
sitions contain modifier bits. The modifier bitsspecify to the device how the command is to he executed.
They may cause, for example, the device to compare
data rcceived during a write operation with data pre
viously recorded, and they may specify such con
ditions as recording density and parity. For the control
command, the modifier bits may contain thc order
code specifying the control function to be performed.
The meaning of the modifier bits depends on the type
ofI/O device and is specified in the SRL publication
for the device.
The command code assignment is listed in the fo]
lowing table. The symhol x indicates that the bit po
sition is ignored; m identifies a modifier hit.CODE X X X X 0000 MMMM 0100 x x x x 1000 MMMM 1100 MMMMMMOI MMMMMM10
MMMMMM11COMMAND Invalid
Sense
Transfer in channelRead hackward \Vritc
Read
Control
Whenever the channel detects an invalid command
code during the initiation of a command, the pro
gram-check condition is generated. When the first ccw
designated by the CAW contains an invalid command
code, the status portion of the csw with the program
check indication is stored during execution ofSTART I/O. When the invalid code is detected during com
mand chaining, the new operation is not initiated, and
an interruption condition is generated. The command
code is ignored during -data chaining, unless it speci
fies transfer in channel.
Definition ofStorage Area
The main-storage area associated with anI/O oper
ation is defined by ccw's. A ccw defines an area by
specifying the address of the first eight-bit hyte to be
transferred and the number of consecutive eight-bit
bytes contained in the area. The address of the first
hyte appears in the data-address field of the ccw. The
number of bytes contained in the storage area is spe
cified in the count field.
In write, read, control, and sense operations storage
locations are used in ascending order of addresses. As
information is transferred to or from main storage, the
content of the address field is incremented, and the
content of the count field is decrementcd.. The read
backward operation causes data to be placed in stor-Input/Output Operations 97
the chain-data flag is off, specifics chaining of com
mands. It causes the operation specified by the com
mand code in the next ccw to be initiated on normal
completion of the current operation. Whcn bit 33 is
zero or when the CD flag is one, the next ccw docs not
specify a new command.
Suppress-Length-Indication Flag: Bit 34 controls
whether an incorrect length condition is to be indi
cated to the program. When this bit is one and the CD
flag is off in the last ccw used, the incorrect-length in
dication is suppressed. If the ccw has the cc flag on,
command chaining takes place. Absence of the
be notified of the incorrect-length condition when it
occurs.
of transfer of information to storage during a read,
read-backward, or sense operation. When bit 35 is
zero, normal transfer of data takes place.
Program-Controlled-Interruption Flag: Bit 36,
when one, causes the channel to generate an inter
ruption condition upon fetching the ccw. When bit 36
is zero, normal operation takes place.
Count: Bits 48-63 specify the number of eight-bit
byte locations in the storage area designated by the
ccw.
Bit positions 37-39 of every ccw other than one spe
cifying transfer in channel must contain zeros.
condition. When the first ccw designated by the CAW
does not contain the required zeros, thc
is not initiated, and the status portion of the csw with
the program-check indication is stored during execu
tion of
data chaining causes the
terminate the operation. When the absence of these
zeros is detected during command chaining, the new
operation is not initiated, and no interruption condition
is generated.
The content of bit positions
ignored.
nel and the
The two low-order bits or, when these bits are
tify the operation to the channel. The channel dis
tinguishes among the following four operations:
Input forward (read, sense)
Input backward (read backward)
Branching (transfer in channel)
The channel ignores the high-order bits of the com
mand code.
Commands that initiate
read backward, control, and sense) cause all eight bits
of the command code to be transferred to the
vice. In these command codcs, the high-order bit po
sitions contain modifier bits. The modifier bits
They may cause, for example, the device to compare
data rcceived during a write operation with data pre
viously recorded, and they may specify such con
ditions as recording density and parity. For the control
command, the modifier bits may contain thc order
code specifying the control function to be performed.
The meaning of the modifier bits depends on the type
of
for the device.
The command code assignment is listed in the fo]
lowing table. The symhol x indicates that the bit po
sition is ignored; m identifies a modifier hit.
MMMMMM11
Sense
Transfer in channel
Read
Control
Whenever the channel detects an invalid command
code during the initiation of a command, the pro
gram-check condition is generated. When the first ccw
designated by the CAW contains an invalid command
code, the status portion of the csw with the program
check indication is stored during execution of
mand chaining, the new operation is not initiated, and
an interruption condition is generated. The command
code is ignored during -data chaining, unless it speci
fies transfer in channel.
Definition of
The main-storage area associated with an
ation is defined by ccw's. A ccw defines an area by
specifying the address of the first eight-bit hyte to be
transferred and the number of consecutive eight-bit
bytes contained in the area. The address of the first
hyte appears in the data-address field of the ccw. The
number of bytes contained in the storage area is spe
cified in the count field.
In write, read, control, and sense operations storage
locations are used in ascending order of addresses. As
information is transferred to or from main storage, the
content of the address field is incremented, and the
content of the count field is decrementcd
backward operation causes data to be placed in stor-