The Art of Assembly Language, 2nd Edition

4.31 Unions

A record definition assigns different offsets to each field in the record according to the size of those fields. This behavior is quite similar to the allocation of memory offsets in a var or static section. HLA provides a second type of structure declaration, the union, that does not assign different addresses to each object; instead, each field in a union declaration has the same offset—0. The following example demonstrates the syntax for a union declaration:

type
     unionType:
          union
               << Fields (syntactically identical to record declarations) >>
          endunion;

You access the fields of a union exactly the same way you access the fields of a record: using dot notation and field names. The following is a concrete example of a union type declaration ...