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6.
When all information exchanges called for by a single
I/o command doubl eword have been compl eted, the
lOP uses the command counter to obtain the next com-
mand doubl eword for execution. This process continues
until all such command doubl ewords associated with the
I/O sequence have been executed.
lOP COMMAND DOUBLEWORDS
All lOP command doublewords (except Transfer in Channel
and Stop) are assumed to be in the following format:
ORDER
Bit positions
0
through
7
of the command doubleword con-
tain the I/O order for the device controller or device. The
I/o orders are shown below. t Bits represented by the letter',
"M" specify orders or special conditions to the device and
are unique for each type of device.
Bit positions
o
1 2 3 4 5 6 7
MMMMM
MMMMM
MMMMM
M M M M 0
MMMMl
M 0
Ml
M 1
1 0
1 0
1
o
1
o
o
Order
Write
Read
Control
Sense
Read Backward
Write. The Write order causes the device controll er to in-
itiate an output operation. Bytes are read in an ascending
sequence from the memory location specified by the memory
byte address field of the command doubl eword. The output
operation continues until thedevice signals "channel end",
or until the byte count is reduced to 0 and no further data
chaining is specified. Channel end occurs when the device
has received all information associated with the output op-
eration, has completed all checks, and no longer requires
the use oflOPfacilities for the operation. Data chaining
is described on the following page.
Read. The Read order causes the device controller to initi-
ate an input operation. Bytes are stored in core memory in
an ascending sequence, beginning at the location specified
by the memory byte address field of the command double-
word. The input operation continues until the device signals
channel end, or until the byte count is reduced to 0 and no
further data chaining is specified. Channel end occurs when
the device has transmitted all information associated with
the input operation and no longer requires the use of lOP
faci I i ti es for the operati on.
tNot all I/o devices recbgnize all these orders. See the
particular XDS SIGMA peripheral reference manual for
orders appl icable to that device.
90
lOP Command Doublewords
Control. The Control order is used to initiatespecial oper-
ations by the device. For magnetic tape, it is used to issue
orders such as rewind, backspace record, backspace fi Ie,
etc. Most orders can be specified ':>y the M bits of the
Control order; however, if additional information is re-
quired for a particular operation (e.g., the starting ad-
dress of a disk-seek), the memory byte address field of the
command doubleword specifies the starting address of the
bytes that are to be transmitted to the device controller for
the additional information. When all bytes necessary for
the operation have been transmitted, the device controller
signals channel end.
Sense. The Sense order causes the device to transmit one or
more bytes of information, describing its current state. The
bytes are stored in core memory in an ascending sequence,
beginning with the address specified by the memory byte ad-
dress field ofthecommanddoubleword. The number of bytes
transmitted is a function of the device and the condition it
describes. The Sense order can be used to obtain the cur-
rent sector address from a disk unit.
Read Backward. The Read Backward order (for devices that
can execute it) causes the device to be started in reverse,
and bytes to be transmitted to the lOP for storage into core
memory in a descending sequence, beginning at the location
specified by the memory byte address field of the command
doubleword. In all other respects, Read Backward is iden-
tical to Read, inc! uding reducing the byte count with each
byte transm itted.
The Transfer in Channel command doub leword is assumed to
be in the following format:
Transfer in Channel. The Transfer in Channel command is exe-
cutedwithin the lOP, and it has no direct effect on any of
the I/o system el eme"ts externa I to the addressed lOP. The
primary purpose of Transfer in Channel is to perm it branch-
ing within the command list so that the lOP can, for exam-
ple, repeatedly transmit the same set of information a num-
ber of times. When the lOP executes Transfer in Channel,
it loads the command counter for the device controller it is
currently servicing with the command doubleword address
field of the Transfer in Channel command, loads the new
command doubleword specified by this address into the lOP
registers associated with the device controller, and then
executes the new command. (Bit positions
0-3,
and
32-63
of the command doubleword for Transfer in Channel are ig-
nored.) Transfer in Channel thus allows a command list to
be broken into noncontiguous groups of commands. When
used in conjunction with command chaining, Transfer in
Channel facilitates the control of devices such as unbuffered
card punches or unbuffered line printers. The current flags
(see "Flags" below) are not al tered during th is command;
thus the type of chain ing called for in the previous com-
mand doubleword is retained until changed by a command
doubleword following Transfer in Channel.
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