Channel
Address
Word
(CAW)
Channel
Block
Address
(CBA)
Channel
Command
Word
(CCW)
Input/Output
Operation
• The Channel Address Word (CAW) is used
by
the
Start
Device in-
struction (see
Privileged Instructions section),
and
specifies the address
of
the first Channel Command Word (CCW) used to control
the
operation
of
the
input/output
device.
If
the
Memory
Protect
option is installed, the
memory protection key
must
also be stored in
the
CAW before a
Start
Device instruction is issued.
The CAW
must
be stored in main memory locations
72
through
75
before executing a
Start
Device instruction
and
has
the
following
format:
Key
0000
Address
of
CCW
o 3 4 7 8
31
Bit
Positions 0 through 3 contain
the
memory protection key.
It
is
used to ensure
that
data
is not being
transferred
to
a protected memory
area.
If
the Memory
Protect
option is not installed, these bits
must
be zero.
Bit
Positions
4-
through 7
are
reserved
for
future
expansion.
Bit
Positions 8 through 31 contain the main memory address
of
the
initial channel command word .
• The Channel Block Address (CBA) is used by the
Start
Device instruc-
tion (see
Privileged Instructions Section),
and
specifies
that
the
Start
Device Operation was initiated (CC =
0).
In
this event,
the
contents
of
the CBA
are
loaded into the selector channel registers
as
follows:
Selector No.
1
2
3
4
Scratch Pad
Word Address
36
66
76
A6
If
mUltiplexing, the contents
of
the
CBA
are
loaded into sub-channel
registers
XXOO
- XX03, where
XX
is
the
device number. The CBA
must
be stored in main memory locations 76
through
79 before executing a
Start
Device instruction
and
has the following
format:
Reserved
Address
of
Channel Control Block
o 7 8
31
Bit
Positions 8
through
31
contain the main memory address
of
the
Channel Control Block.
Note: The CCW and the CBA addresses
are
24 bits
and
direct
(not
translatable) .
• The Channel Command Word (CCW) supplies the information
for
controlling
the
operation
of
the
input/output
device. This
information
must
be stored in main memory by the
program
before a
Start
Device
instruction is issued. The CCW consists
of
two 32-bit words in main
memory
that
must
be aligned on a double word boundary. The CCW
has
the following
format:
40
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