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 10–7
The Header of an MDO
Variable
All MDO variables have a header. An MDO variable header identifies the beginning
of an MDO variable. A header consists of two parts: a length field and a key field.
Figure 10-1 shows the structure of the header in an MDO variable mapped by the map
$MSG.
Figure 10-1. Header in an MDO Variable
len
4 byte
$MSG
(02)
4 byte
Element Length
Length of
Key Field
Element Key Value
Element Symbolic Name
Length
Field
Key
Field
Header
Length of
Length Field
004
The Length Field of an MDO Variable Header
The length field of an MDO variable header contains the length of the whole MDO
variable. Messages vary in length. Accordingly, the value of this field can vary from
MDO variable to MDO variable. This is reflected by the letters "len" in Figure 10-1.
The length of the length field itself is included in the length of an MDO variable.
Figure 10-1 shows this is fixed at 4 bytes in an MDO variable mapped by the map
$MSG.
When an MDO variable is created, Mapping Services automatically calculates the total
length of the variable and places it in the length field of the MDO variable header.