The statements 

PARAMETER_TABLE   DW     PAR1
                  DW     PAR2
                  DW     PAR3
would cause the offsets of PAR1, PAR2, and PAR3 to be stored as shown in Fig. 3-58(a). PAR1, PAR2, and PAR3 may be variables or labels. Statements such as 
INTERSEG_DATA   DD     DATA1
                DD     DATA2
could be used to store both the offsets and segment addresses, as shown in Fig. 3-58(b).

Figure 3-58 Use of DW and DD statements to preassign addresses.

(a) DW statement(b) DD statement

The assembler uses the type attribute to determine whether the machine instruction is to operate on a byte or word (i.e., the w-bit is to be set to 0 or 1). For example, given 

        MOV    OPER1,0
        MOV    OPER2,0
         .
         .
         .
OPER1   DB     ?,?
OPER2   DW     ?,?
the w-bit is set to 0 in the first MOV instruction and to 1 in the second.

The LABEL directive, which

Variable    LABEL    Type
causes the variable to be typed and assigned to the current offset. The directives 
BYTE_ARRAY   LABEL  BYTE
WORD_ARRAY   DW     50 DUP(?)
would assign both BYTE_ARRAY and WORD_ARRAY to the same location, the first byte of a 100-byte block. The instruction 
MOV     WORD_ARRAY+2,0
would set the third and fourth bytes of the block to 0 and 
MOV     BYTE_ARRAY+2,0
would set the third byte to 0.
Table of Contents | Next page | Previous page

This is page 43.