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", 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" 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
PHASE
An
I/O
operation may be 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" condition
was
detected.
3. Aborted when a conditional "unusual end" condition
was
detected
whi
Ie
the
HTE
control flag was coded
as a
1.
4. Completed as specified by the command list but with
an
"unusual end" condition.
5.
Completed as specifi ed 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
I/O
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
an
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
ll
condition, or when the
lOP
or
any
device
controller
detects
an lIunusual 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 Rewind 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-handling 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 peripheral equipment reference manual for
further
detai
Is.
)
Since a multiple number of
I/O
interrupt requests may
pre-
vail simultaneously
{one
per
each
de
vi
ce
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
interrupt was requested as
the
result of a
zero
byte
count
or
II
channel end"
condition,
and
data
chaining
or command
chaining
is
specified,
the
I/O
operation may
continue
as
specified by
the
command
list.)
I/O
Operation
Phases
151
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