Termination Phase - Xerox 550 Reference Manual

Depending upon the characteristics of the device, command
chaining may also be used to perform different operations
on either different but consecutive records or upon the same
record (e. g., a magnetic tape unit may be programmed to
alternately read or write consecutive records or to read the
same record backwards after writing). Refer to an appro-
priate Xerox peripheral equipment reference manual for
further detai Is.
If command chaining is specified, the device controller
causes the lOP to fetch the next operational lOCO, as de-
scribed under "Fetching Phase ll
,
when the device signals
"channel end" {signifying that it is ready to accept and
execute another order}. In the process of fetching the next
operational lOCO, the lOP may fetch and execute a con-
trol lOCO containing a Transfer in Channel command with-
out affecting the continuity of the I/O operation {i. e. ,
transparent to the device controller/device}; however, the
fetching of the next operational lOCO is not transparent
to the device controller/device. The process of automat-
ically fetching the next operational lOCO because data
chaining and/or command chaining is specified in the cur-
rent lOCO permits an I/O operation to continue normally
unti I an lOCO is executed in which both chaining flags
are coded as zeros (the last lOCO of the last record).
If data chaining and command chaining are both specified
within an lOCO, data chaining is performed if the byte
count of the current lOCO is reduced to zero before the
device generates "channel end"; command chaining is per-
formed if the device generates "channel end" before the
byte count is reduced to zero.
During the execution phase, an I/O interrupt may also be
requested each time a "channel end ll occurs if the Interrupt
at Channel End (ICE) flag is coded as a 1. Thus, if com-
mand chaining is specified, the lOP may request an I/O
interrupt without interfering with the process of fetching
the next operational lOCO.
TERMINATION P~ASE
An I/O operation maybe terminated in one of the following
manners:
1.
Aborted at any time because the BP executed either
an HIO or RIO instruction.
2. Aborted when an unconditional "unusual end ll condition
was detected.
3. Aborted when a conditional "unusual end" condition
was detected whi Ie the HTE control flag was coded
os c
1.
4. Completed as specified by the command list but with
an "unusual end" condition.
134 I/O Operation Phases
5. Completed as specified by the command list.
6. Aborted whenever a SUPER RESET, SYSTEM RESET,
or I/O RESET command is entered from the System Con-
trol Console (SCC).
The progress of an 1/0 operation, including the termination,
may be ascertained by evaluating the status information
returned for I/O instructions, as described in Chapter 3.
Depending upon programming considerations, these I/O
instructions may be executed either singly or as part of an
I/O handling routine and either imperatively at logical
poi nts of a BP-executed program or on a,n
II
as needed ll
basis when an I/O interrupt is requested by an lOP or de-
vice controller. Normally, an I/O interrupt is requested
whenever a critical or significant event occurs within any
I/o subchannel, device controller, or device. Typically,
an I/O interrupt may be requested when the byte count of
any lOCO is reduced to zero, whenever any device detects
a "channel end" condition, or when the lOP or any device
controller detects an "unusual end" condition, providing
the appropriate control flag {IZC, ICE, and IUE} is coded
as a 1.
Note: An I/O interrupt may also be requested by certain
devices, e. g., a magnetic tape unit may be able
to execute a Rewi nd and Interrupt order and other
devices may request an I/O interrupt when execut-
ing a Stop order in which bit 0 is coded as a 1.
Refer to an appropriate Xerox peripheral reference
manual for further detai Is.
Once an I/O interrupt request has been made by a device,
that device, device controller, and I/O subchannel remain
in an interrupt pending condition unti I the interrupt request
is acknowledged, reset, or cleared.
Normally, an I/O interrupt request is acknowledged by
the BP executing an AIO instruction, as part of an I/O
interrupt-handli ng routine; reset by the BP executing either
an HIO or an RIO instruction; or for certain devices cleared
automatically, as a function of time. Refer to an appro-
priate Xerox periphera! equipment reference manuel
fCi
further detai Is. )
Since a multiple number of I/O interrupt requests may pre-
vail simultaneously {one per each device controller} and
all requests are serviced by a common I/O interrupt level
(location X ' 5C'), the BP normally acknowledges an I/O
interrupt request based on the priority that prevai Is within
the interrupt system, the lOPs, and the I/O subchannels
within an MIOP, if applicable. An interrupt pending con-
dition prevents a new I/O operation from being initiated
by an SIO instruction on a particular subchannel but does
not affect the current I/O operation. (That is, if an I/O
intellupt was requested as the resuit of a zero byte count or
"channel end" condition, and datt" chaining or command
chaining is specified, the I/O operation may continue as
specified by the command list.)