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.