not interrupted, and any pending attention and de­
vice-end conditions in these devices are not reset.
When any of the following conditions occurs, HALT I/O causes the status portion, bit positions 32-47, of
the csw at location 64 to be replaced by a new set of
status bits. The status bits pertain to the device ad­
dressed by instruction. The contents of the other fields
of the csw at location 64 are not changed. The extent
of data transfer and the conditions of termination of
the operation at the subchannel are provided in the
csw associated with the termination.
1. The device on the addressed subchannel current­
ly involved in data transfer in the multiplex mode has
been signaled to terminate the operation. The csw
contains z,eros in the status field.
2. The addressed subchannel on the multiplexor
channel is working, and no burst operation is in prog­
ress, but the control unit or the I/O device is executing
a type of operation or is in such a state that it cannot
accept the halt-I/o signaI. The device has not been
signaled to terminate the operation, but the subchan­
nel has been set up to signal termination to the device
the next time the device requests or offers a byte of
data. The csw unit-status field contains the busy and
status-modifier bits. The channel-status field contains
3. The channel detected an equipment malfunction
during the execution of HALT I/O. The status bits in
the csw identify the error condition. The state of the
channel and the progress of the I/O operation are un­
When the subchannel on the multiplexor channel is
shared and no burst operation is in progress, HALT I/O causes the operation to be terminated as long as the
instruction is addressed to a device on the currently
working control unit. If another device is addressed, a
malfunction has occurred, or the operator has changed
the state of the operating control unit, no device may
recognize the address. If the device appears not oper­
ational during execution of HALT I/O, condition code
3 is set, and the subchannel is set up to signal ter­
mination to the device the next time the device offers
or requests a byte of data.
Resulting Condition Code:
o Channel and subchannel not working
1 csw stored
2 Burst operation terminated
3 Not operational Program Interruptions: Privileged operation.
Programming Note
The instruction HALT I/O provides the program a
means of terminating an I/O operation before all data
specified in the operation have been transferred. It
permits the program to immediately free the selector
channel for an operation of higher priority. On the
multiplexor channel, HALT I/O provides a means of
controlling real-time operations and permits the pro­
gram to terminate data transmission on a communi­
cation line.
Test Channel TCH 51 9F 78 1516 1920 31
The condition code in the psw is set to indicate the
state of the addressed channeI. The state of the chan­
nel is not affected, and no action is caused. The in­
struction TEST CHANNEL is executed only when the CPU is in the supervisor state.
Bit positions 21-23 of the sum formed by the addi­
tion of the content of register Bl and the content of
the Dl field identify the channel to which the instruc­
tion applies. Bit positions 24-31 of the address are
The instruction TEST CHANNEL inspects only the state
of the addressed channeI. It tests whether the channel
is operating in the burst mode, is aware of any out­
standing interruption conditions from its devices, or
is not operationaI. When none of these conditions
exists, the available state is indicated. No device is
selected, and, on the multiplexor channel, the sub­
channels are not interrogated.
Resulting Condition Code:
o Channel available
1 Interruption pending in channel
2 Channel operating in burst mode
3 Channel not operational
Input/Output Operations 95
Execution of Input/Output Operations
The channel can execute six commands:
Read backward
Transfer in channel
Each command except transfer in channel initiates
a corresponding I/O operation. The term "I/O oper­ ation" refers to the activity initiated by a command in
the I/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 of 110 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 one I/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 with START I/O. It appears at location 72. The
channel refers to the CAW only during the execution
of START 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
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:
96 Protection Key: Bits 0-3 form the storage protection
key for all commands associated with START 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, the I/O operation is not
initiated. Channel Command Word
The channel command word (ccw) specifies the com­
mand to be executed and, for commands initiating I/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 a START 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 of START 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 78 Flags Count 32 36 37 3940 4748 63
The fields in the ccw are allocated for the following
Command Code: Bits 0-7 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.
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