Xerox 560 Reference Manual page 150

Orders that are executed by a specific type of device are
listed and described in the appropriate Xerox peripheral
equi pment reference manua I.
When an operational 10CD is fetched by the lOP, the con-
tent of the order field, if required, is loaded into an order
register within the device controller/device. If two or
more 10CDs are required to define a logical record (as de-
scribed under "Data Chain Flag"), the order obtained from
the first 10CD prevai Is for all subsequent 10CDs within that
logical record and any orders contained within the subsequent
10CDs are ignored.
MEMORY BYTE ADDRESS
This 22-bit field (bit positions 10-31), if required, is
coded with the initial memory byte address for the I/o op-
eration that wi II be performed when the current 10CD is
executed. When the 10CD is fetched by the lOP, the con-
tent of the memory byte address field is loaded into a
memory byte address register within the appropriate I/O
subchannel of the lOP. Thereafter, the content of the
memory byte address register is incremented (or decremented
during Read Backward operations) by one for each byte of
data or information transmitted, even though access to main
memory may be inhibited (as described under "Skip Flag")
or the data is rejected by a memory unit (as described under
ItWrite Keylt).
Depending upon the characteristics of the I/O device, the
content of bit positions 10-31 may either be ignored (e. g.,
HRewind" order for magnetic tape units) or specify memory
byte locations that contain supplemental control information
(e. g., starting address for a disk seek operation). Refer to
an appropriate Xerox peripheral equipment reference man-
ual for further detai Is.
FLAGS
Each operational IOCD contains eight control flogs (bif
positions 32-39). As described below, each control flag
is coded to specify a particular control function that may
be performed by the lOP either during or at the end of the
current 10CD.
Data Chain Flag (Bit Position 32). Coding of the data chain
flag is dependent upon the number of 10CDs required to
defi ne the data transfers for a logi ca I record. If two or
more 10CDs are required (e. g. , to perform a "gather-write"
or a "scatter-read It operation), the data chain flag of each
operational IOCD, except the last 10CD, must be coded as
a
i.
The data chain fiag of the iast iOCD or the oniy
10CD (if the record is defined by a single 10CD) is coded
as O. If data chaining is specified and no error conditions
are encountered, the lOP wi
II
automatically fetch the next
operational 10CD when the byte count (described later) of
the current 10CD is reduced to zero. {Note: The lOP may
also fetch and execute a control 10CD containing a Transfer
144 Input/Output Processor (IO P) Fundamenta Is
in Channel command, as described later, before fetching
the next operational 10CD.) As a result of fetching the
next operational 10CD, all parameters, except the I/o
order, are updated and the device controller/device con-
tinue to operate as if the I/o operation were defined by
a single 10CD (i. e., the data chain operation is transparent
to the device controller/device). If data chaining is not
specified, the lOP wi II generate a "count done" signal when
the byte count of the current 10CD is reduced to zero. The
"count done ll signal indicates that the lOP has completed
all data transfers for the current logical record. However,
as described under
II
Interrupt on Channel End Flag ll
,
the
I/o order is not completed until the device signals a IIchan-
nel end".
Interrupt at Zero Byte Count Flag (Bit Position 33).
If
an
I/o interrupt is to be requested when the byte count of the
current 10CD is reduced to zero, the Interrupt at Zero
Byte Count (IZC) flag must be coded as a 1.
If
the I/o
interrupt level within the interrupt system (location XISC')
is armed, enabled, and not inhibited, the request will be
processed by the BP in accordance with the priority that
prevai Is within the interrupt system, the lOPs, and the I/o
subchannels within an MIOP. The occurrence of an I/o
interrupt because of a zero byte count condition is reported
as status information (bit position 10 of register R) when the
BP executes an AIO instruction (normally part of the I/o
interrupt handling routine). The I/O interrupt request may
be processed without interfering with the I/o operation.
(Note: An I/O interrupt may be requested at "channel end"
or on "unusual end lt condition, as described later.)
Command Chain Flag (Bit Position 34). Command chaining
permits an I/O device to execute a multiple number of
orders relating to the same I/o operation in a consecutive
manner (e. g., when reading a multi-record file, the I/o
device may automatically receive a new Read order upon
completing the current Read order without the BP execut-
ing another SIO instruction). Command chaining, if re-
quired, is specified by coding the command chain flag as
a 1 in the 10CD of each record, except the last.
If
command chaining is specified, the lOP wi
II
fetch the
next operational 10CD when the devi'ce signals a "channel
end" unless terminated by an lIunusual end" condition. As
a result, new parameters are stored in the appropriate
registers within the I/O subchannel and a new I/O order
is received by the device controller/device.
Thus, an lOP wi
II
automatically access main memory and
fetch the next operational 10CD if either data chaining or
command chaining is specified. If data chaining and com-
mand chaining are both specified in the same command
doubleword, a data chaining operation wi" be performed if
the byte count is reduced to zero before the devi ce si gna Is
a "channel end" and a command chaining operation will be
performed if a "channel end" occurs before the byte count
is reduced to zero.
If
neither data chaining or command
chaining is specified, the I/o operation is completed when
the device signals a "channel end". Note that command
chaining is inhibited by "unusual end".