HP DDL D40 Reference Manual page 207

SPI Tokens
Derived from the SPI null value of a group definition that contains the
elementary item.
Implicitly specified by default; the default value for SPI-NULL is 255.
The null value specification for a group of bit fields that share the same byte or
word is generated as one contiguous block having an SPI-NULL value of 255
following the version number.
You can specify a REDEFINES clause in the definition of an extensible structured
token, but redefined fields have the same SPI null value as the fields they overlay.
If you include comments in your token map definition, DDL issues a warning
message and does not save the comments.
SPI considers a field to contain an SPI null value if every byte in the field contains
the SPI null value for the field. You use the SSNULL operating system procedure to
set each field of the structure to its specified SPI null value. For a description of the
SSNULL procedure, see the Distributed Systems Management (DSM) Manual.
For the SPI null value to indicate the presence or absence of a value in its
associated field, the SPI null value must not be a legitimate value for the field. If
every possible value of a field is legitimate, then an SPI null value cannot be used
to indicate the presence or absence of a value. In such a case, you have two
alternatives:
Indicate the presence or absence of a non-null value in the field by an is-
present field. An is-present field is a Boolean field that can be set to -1 to
indicate that the field has a value (is present), or set to 0 to indicate the field
value is null (is not present). Note that the field must still have an explicit or
implicit null value.
Make the field larger. For example, if a field is a 16-bit integer and all 16-bit
values are valid for the field, you could define the field as a 32-bit integer.
Lengthening the field enables you to choose an SPI null value that creates a
value in the 32-bit integer that is not one of the valid values for the 16-bit
integer.
For more information on using versions in extensible structured tokens, see the
SPI Programming Manual and the SPI Common Extensions Manual.
Versions for Bit Fields. The following points are guidelines for specifying versions for
bit fields:
Bit fields that share the same byte must have the same version number in the
token map. The version number applies to the entire byte. If a bit field extends
across two bytes within a word, the version number of that field applies to the
entire word.
The version number for a bit filler depends on the filler's position in a word and the
length of the filler.
Data Definition Language (DDL) Reference Manual—426798-002
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TOKEN-MAP Statement