Syntax

[utag]       .union        [expr]

[mem₀ ]    element      [expr₀]
[mem₁ ]    element      [expr₁]
.               .               . 

[mem_n ]     .tag utag     [expr_n ]
.               .               . 

[mem_N ]    element      [expr_N]

[size]       .endunion

label        .tag             utag

Description

The .union directive assigns symbolic offsets to the elements of alternate data structure definitions to be allocated in the same memory space. This enables you to define several alternate structures and then let the assembler calculate the element offset. This is similar to a C union. The .union directive does not allocate any memory; it merely creates a symbolic template that can be used repeatedly.

A .struct definition can contain a .union definition, and .structs and .unions can be nested.

The .endunion directive terminates the union definition.

The .tag directive gives structure or union characteristics to a label, simplifying the symbolic representation and providing the ability to define structures or unions that contain other structures or unions. The .tag directive does not allocate memory. The structure or union tag of a .tag directive must have been previously defined.

Following are descriptions of the parameters used with the .struct, .endstruct, and .tag directives:

• The utag is the union's tag. is the union's tag. Its value is associated with the beginning of the union. If no utag is present, the assembler puts the union members in the global symbol table with the value of their absolute offset from the top of the union. In this case, each member must have a unique name.
• The expr is an optional expression indicating the beginning offset of the union. Unions default to start at 0. This parameter can only be used with a top-level union. It cannot be used when defining a nested union.
• The mem_n/N is an optional label for a member of the union. This label is absolute and equates to the present offset from the beginning of the union. A label for a union member cannot be declared global.
• The element is one of the following descriptors: .byte, .char, .int, .long, .word, .xldouble, .half, .short, .string, .float, and .field. An element can also be a complete declaration of a nested structure or union, or a structure or union declared by its tag. Following a .union directive, these directives describe the element's size. They do not allocate memory.
• The expr_n/N is an optional expression for the number of elements described. This value defaults to 1. A .string element is considered to be one byte in size, and a .field element is one bit.
• The size is an optional label for the total size of the union.

These examples show unions with and without tags.

Example 1

       1              
       2                           .global employid
       3                xample     .union            ; utag
       4          0000  ival       .int              ; member1 = int
       5          0000  fval       .float            ; member2 = float
       6          0000  sval       .string           ; member3 = string
       7          0002  real_len   .endunion
       8              
       9 00000000       employid   .usect ".ebss", real_len  ; allocate memory
      10              
      11                employid   .tag xample       ; name an instance
      12              
      13 00000000 08A1-            ADD AR1, #employid.ival
         00000001 0000

Example 2

       1              
       2                      .union          ; utag
       3        0000  x       .long           ; member 1= long
       4        0000  y       .float          ; member 2 = float
       5        0000  z       .int            ; member 3 = int
       6        0002  size_u  .endunion       ; size_u = 2