NET/MASTER Network Control Language (NCL) Programmer's Guide

The Structure of Mapped Data Object (MDO) Variables
Working With Mapping Services
106160 Tandem Computers Incorporated 10–9
The Key Field of an MDO Variable Header
The key field of an MDO variable header uniquely identifies the map used to access
the MDO variable. The value contained in the key field is the name of the map that
Mapping Services uses to interpret the MDO variable. The name of the map in
Figure 10-1 is $MSG. This is called a symbolic name.
The symbolic name of a map is represented internally in an MDO variable. The
internal representation of a symbolic name is called a key value. Figure 10-1 shows
the key value for $MSG is (the decimal number) 2. In an MDO variable, when
Mapping Services encounters the key value of 2 as part of the MDO variable header,
Mapping Services interprets it to mean: "Use the map $MSG when analyzing this
MDO variable." Every MDO variable described by the map $MSG has the key value of
2 as part of its header.
The length of the key field is included in the length of an MDO variable. Figure 10-1
shows this is fixed at 4 bytes in an MDO variable.
When an MDO variable is created, Mapping Services automatically encodes the name
of the map used to interpret the MDO variable in the key field of the MDO variable
header. When you refer to an MDO variable from an NCL procedure, Mapping
Services uses the map name to interpret the structures within the variable.
The key field of an MDO variable header contains internal Mapping Services data.
You cannot directly access the key field of an MDO variable header from an NCL
procedure. You can use the SHOW MAPS command to display the symbolic names
and corresponding key values of map definitions, as the following screen shows:
(07:34) --------------------- OPERATOR CONTROL SERVICES ----------------------
SHOW MAPS
NNM0256 MAP-NAME MAP-KEY USE-CNT STATUS
NNM0257 $MAP 00000030 0 NAMEONLY
NNM0257 $MSG 00000002 0 LOADED
NNM0257 $NCL 00000001 0 LOADED
NNM0257 $SEC 00000010 1 LOADED
NNM0257 $SPI 00000077 1 LOADED
NNM0999 *END* 5 MAPS DISPLAYED
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