DXD instruction
The DXD instruction identifies and defines an
external dummy section.
 >>-symbol--DXD--+--------------------+--type-------------------->
'-duplication_factor-'
>--+----------------+--+----------+--+-------------------+-----><
'-type_extension-' '-modifier-' | .-,-------------. |
| V | |
'---nominal_value-+-'
|
- symbol
- is
an external symbol which is
one of the following:
- An ordinary symbol
- A variable symbol that has been assigned a character string with
a value that is valid for an ordinary symbol
- duplication_factor
- is the duplication factor subfield equivalent to the duplication
factor subfield of the DS instruction.
- type
- is the type subfield equivalent to the type
subfield of the DS instruction.
- type_extension
- is the type extension subfield equivalent to the type extension
subfield of the DS instruction.
- modifiers
- is the modifiers subfield equivalent to the modifiers
subfield of the DS instruction.
- nominal_value
- is the nominal-value subfield equivalent to the nominal-value
subfield of the DS instruction.
The nominal value is optional. If specified, it is not
generated.
The DXD instruction can be used anywhere in a source module,
after the ICTL instruction.
In order to reference the storage defined by the external
dummy section,
the ordinary symbol denoted by
symbol
must appear in the
operand of a Q-type constant. This symbol represents the address of the
first byte of the external dummy section defined, and has a length
attribute value of 1.
The subfields in the operand field (duplication factor, type,
type extension, modifier, and nominal value) are specified in the same
way as in a DS instruction. The assembler computes the amount of storage
and the alignment required for an external dummy section from the area
specified in the operand field. For more information about how to
specify the subfields, see DS instruction.
For example:
A DXD CL20 20 bytes, byte alignment
B DXD 3F,XL4 20 bytes, fullword alignment
C DXD LQ 16 bytes, quadword alignment
The linker uses the information provided by the
assembler to compute the total length of storage required for all
external dummy sections specified in a program.
Notes:
- The DSECT instruction also defines an external dummy section, but
only if the symbol in the name field appears in a
Q-type offset constant
in the same source module. Otherwise, a DSECT instruction defines a
dummy section.
- If two or more external dummy sections for different source modules
have the same name, the linker uses the most restrictive
alignment, and the largest section to compute the total length.
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