Variable play the same role in assembly language that they do in high-level languages. Each variable
has a data type and is assigned a memory address by the program .The data -defining pseudo-op can
be used to set aside one or more data items of the given type .
in this section we use D B and D W to define byte variable , word variable ,and arrays of byte ,word are used in chapter 18 in connection with multiple - precision and noninteger operations