3-10 DIRECTIVES AND OPERATORS
Assembler instructions are translated into machine language instructions and correspond to executable statements in high-level language programs. Just as high-level language programs must have nonexecutable statements to preassign values, reserve storage, assign names to constants, form data structures, and terminate a compilation, assembler language programs must contain directives to perform similar tasks.
3-10-1 Data Definition and Storage Allocation
Statements that preassign data and reserve storage have the form:
Variable Mnemonic Operand, . . . , Operand ;Comments
where the variable is optional, but if it is present it is assigned the offset of the first byte that is reserved by the directive. Note that unlike the label field, a variable must be terminated by a blank, not a colon. The mnemonic determines the length of each operand and is one of the following:
- DB (Define Byte) - Each operand datum occupies one byte.
- DW (Define Word) - Each operand datum occupies one word, with its low-order part being in the first byte and its high-order byte being in the second byte.
- DD (Define Double Word) - Each operand datum is two words long with the low-order word followed by the high-order word.
To preassign data the operand must be a constant, an expression that evaluates to a constant, or a string constant. For example,
DATA_BYTE DB 10,4,10H
DATA_WORD DW 100,100H,-5
DATA_DW DD 3*20,0FFFDH |
will cause a sequence of bytes to be preassigned as shown in Fig. 3-55.
Figure 3-55 Typical preassignment of data using the DB, DW, and DD directives.
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