Slide 5.4: Data definition statements

Slide 5.3: Intrinsic data types
Slide 5.5: Data definition statements (cont.)
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Data Definition Statements

        X      SWORD    -20h
        Y      BYTE     10101010b, 'A', 10
               SBYTE    ?

A data definition statement sets aside storage in memory for a variable and may assign a name to the variable. Each data definition has the syntax:

  [ name ]  directive  initializer  [, initializer ] ...

Name
A variable name is a label that marks the offset of a variable from the beginning of its enclosing segment. For example, suppose that X was located at offset 0 in the data segment and consumed 2 bytes of storage. Then Y would be located at offset 2.

Labels are identifiers that act as a place marker for data/instructions.

Identifiers are programmer-chosen names. Some examples are: temp, COUNTER, _expr, $store, or A123 . Rules for creating them are
- They may contain between 1 and 247 characters.
- They are NOT case-sensitive.
- The first character must be either a letter (A..Z, a..z), _, @, ?, or $. Subsequent characters may also be digits.
- An identifier can not be the same as a reserved word.

Reserved words have special meaning and can only be used in their correct context. They can be any of the following:
- Instruction mnemonics, such as ADD, MOV, or SUB
- Directives, such as TITLE or INCLUDE
- Attributes, such as BYTE or WORD
- Operators, such as +, or –
- Predefined symbols, such as @data
A list of the MASM reserved words can be found from here.