Channel Command
Word (CCW)
(Cont'd)
o
Command
Code
Flags
Input/Output
Operation
Address of
First
Data
Byte
or
Address of
Next
CCW
if
Command is a
Transfer
in
Channel
7 8
Reserved
for
Future
Expansion
Byte Count
31
32
36 37
47
48
63
Bit
Positions 0 through 7 contain
the
command code, which specifies
the
operation to be
performed
by
the
I/O
device. (See table 10.)
Table 10. Command Code Operations
Command Code
Operation
0
1 2
3
4 5
6 7
Bit
Position
M
M
M M 0 0 0 1 Sense
M M M
M
M/O
0 1
0
Read
Reverse
M M M
M/B
M/O
0
1
1
Write
M M M
M/B
M/O
1
0
0
Write
Erase
M M
M
M/B
M/O
1
0 1
Read
M M
M
M
0
1
1 1
Write
Control
M M
M
M
1
0 0 1
Transfer
in
Channel
Notes:
1.
Any
command code
other
than
the
ones shown
in
table 10 is illegal
and
must
not
be programmed.
If
this
rule
is violated,
the
'resulting
effect on
the
channel
and
device is unpredictable.
If
one
of
the
legal
commands is issued to a device which is
not
capable
of
accepting
the
operation
(i.e., a
Write
command is issued to a
card
reader),
the
command,
after
being
accepted, is
terminated
by
the
device con-
trol
electronics. A channel
interrupt
occurs
and
the
sense
byte(s)
indicate
the
illegal operation.
2.
The
bit
position designated
as
"B"
indicates
that
the
specified device
is connected
to
the
multiplexor channel
and
the
multiplexor is to
be
operated
in
the
burst
mode.
If
this
position is a 1 bit,
the
multi-
plexor channel
is
locked-on to
the
selected device,
and
the
servicing
of
other
devices connected to
the
mUltiplexor channel is inhibited.
A
burst
mode can only
be
initiated
when
it
is specified
in
the
first
command
of
a chain. Subsequent commands, linked
by
chaining,
cannot
initiate
a
burst
mode. However,
if
the
first command
in
a
chain
specifies
burst
mode, all commands
in
the
chain
are
executed
under
burst
mode conditions.
3.
Bit
positions designated
as
M (modifier) indicate
variations
of
the
operation
and
are
unique to
the
specific
input/output
device.
An
explanation
of
the
commands shown
in
table 10 is
as
follows:
Sense
(0001)
-
Information
is
transferred
from
the
specified input/
output
device control electronics to
main
memory.
The
information
trans-
41
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