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Explanations of Logic Blocks
1.
The entry is in the form of an XCTL macro-instruction issued by the
control program.
2.
Use the GETMAIN macro-instruction to obtain main storage.
3.
Use the store mUltiple instruction (STM).
4.
TO
locate
the
address
of
the data control block (DCB) , use
contents of register 5.
To determine if the DCB is open, test
3
of
the
DCBOFLGS field of the DCB; if this bit is zero, the
has not been opened.
(The symbolic names of all fi¢lds in the
are defined by the DCBD macro-instruction.)
the
bit
DCB
DCB
5.
To
determine
if
a
tape
data
set
is being processed, test the
UCB3TAPE field of the unit control block (UCB); this bit is one for
a tape data set.
The symbolic names of all fields in the
UCB
are
defined
by
the IEFUCBOB macro-instruction.
(The macro-definition
and how to add i t to the macro-library are in the Appendix of
this
chapter.)
The
address
of
the
UCB is contained in the DXDEBUCB
field of the data extent block (DEB) as
defined
by
the
IECDSECT
macro-instruction.
(The macro-definition and how to add it to the
macro-library are in the Appendix of this chapter.)
6.
To determine if nonstandard labels have been
specified,
test
the
JFCBLTYP
field
of
the
job file control block (JFCB); this field
contains
a
hexadecimal
04
when
nonstandard
labels
have
been
specified.
7.
The
final DCB entry in the list of DCB addresses contains a one in
its high-order bit position.
8.
Add 4 to the contents of register 5;
add
8
to
the
contents
of
register 6.
9.
Set the high-order bit to zero in the SRTEDMCT field of the UCB.
10.
To
determine
the
type
of
I/O
operation
specified in the OPEN
macro-instruction, check the bit configuration
of
the
high-order
byte
of
the
DCB
entry
in
the
list of DCB addresses.
The bit
configuration for each type of I/O operation is shown below.
(The
high-order four bits correspond to the disposition of the data set;
the
low-order
four
bits
correspond to the I/O operation itself.
For example, the bit configuration xOll0000 indicates
a
data
set
opened for input whose disposition is LEAVE.)
11.
To
the
mode
(The
DCBD
52
Bits
o
1
x
0
x
0
x
0
2
3
o
1
1
1
o
0
4
5
x
x
x
x
x
x
x
x
x
x
0
0
x
x
x
x l i
x
x
x
x
0
0
x
x
x
x
0
1
x
x
x
x
0
0
x
x
x
x
0
1
6
7
x
x
x
x
x
x
o
0
1
1
1
1
1
1
o
1
o
0
REREAD
LEAVE
Neither REREAD nor LEAVE
INPUT
OUTPUT
INOUT
OUTIN
RDBACK
UPDAT
determine
the mode of the data set, test the high-order bit of
DCBOFLGS field of the DCB.
If this bit is
one,
the
data-set
is
output;
if this bit is zero, the data-set mode is input.
symbolic names of all fields in the DCB
are
defined
by
the
macro-instruction.)
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