Initialization of Aggregate Types

An aggregate data item is a collection of one or more sub-items of possibly dissimilar types that are allocated, initialized, and treated as a single unit. The sub-items usually have unique names, and their positions relative to other sub-items is significant. The assembler provides the ability to define aggregate types through use of the RECORD, STRUCT, and UNION directives.

Initialization of an aggregate data item requires a programming notation that isolates the entire aggregate from surroundings constructs, and denotes the position of each sub-item within the aggregate. The syntax for this construct is as follows:

Aggregate-Initializer:



• Initializer-List] }
  < [Initializer-List] >
  
Initializer-List:
Initializer-Item
Initializer-List, [LineBreak] Initializer-Item

Initializer-Item:
[Scalar-Initializer]

• Aggregate-Initializer]
  
• Array-Initializer]
  

The syntax requires that an Aggregate-Initializer be enclosed in an outer set of braces or angle brackets, but the Initializer-List or individual comma-separated Initializer-Items may be left unspecified, in which case a default initializer value is used. Commas are used to denote the position of each sub-item within the entire aggregate, and nested initializers are allowed to accomodate imbedded occurrences of other aggregates (or vector types, which share the same initializer syntax).

When initializing an instance of a union, the assembler only allows an initializer to be specified for the first field defined in the union type.

Examples

Here are some examples of aggregate initialization:

YES   equ  1NO    equ  0
MAYBE equ -1

BOOL_T typedef sbyte

IDEAS_T struct
  sanctum  BOOL_T ?          ; For scalar data, use the ? operator
  peace    BOOL_T ?          ;   to request an uninitialized value.
  pilzner  BOOL_T ?
IDEAS_T ends

PROBLEM_T struct
   work   BOOL_T  YES        ; Establish default initial values that
   car    BOOL_T  NO         ;   can be inherited when an instance of
   house  BOOL_T  MAYBE      ;   the structure is allocated
PROBLEM_T ends

SOLUTION_T struct
    fixing PROBLEM_T {}      ; Outermost set of braces required even
           IDEAS_T   {}      ;  with unspecified (default) initializers
SOLUTION_T ends

DATA segment
  ProblemWith PROBLEM_T  { NO, , MAYBE }         ; First-level structure
  ThinkOf     SOLUTION_T { { YES, YES, YES },    ; Intializer syntax for
                           {  NO,  NO,  NO } }   ;   imbedded structures
DATA ends

CODE   segment
       assume ds:DATA
       mov al, NO
       or  al, ProblemWith.work
       or  al, ProblemWith.car
       or  al, ProblemWith.house
       jz  exit
       mov ThinkOf.fixing.work, NO     ; References to named fields in
       mov ThinkOf.fixing.car, NO      ;   imbedded structures must be
       mov ThinkOf.fixing.house, NO    ;   fully qualified.
exit:  mov ThinkOf.pilzner, YES        ; Reference to "promoted" field
       ret
CODE   ends
end

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