13. EQU Directive and Symbolic Integer Constants

• Two MASM directives,

      symbol EQU expression
      symbol = expression
  

  generate integer constants. For example,

         .CONST
      column  EQU    80                ; Constant    80 
      row     EQU    25                ; Constant    25
      screen  EQU    column * row      ; Constant  2000
      line    EQU    row               ; Constant    25
  
         .DATA
  
         .CODE
         mov     cx, column
         mov     bx, line
  

• Using symbolic constants instead of undescriptive numbers makes your code more readable and easier to maintain.

• The assembler does not allocate data storage when you use either EQU or = .

• The assembler simply replaces each occurrence of the symbol with the value of the expression.

• Integers defined with the = directive can be redefined with another value in your source code, but those defined with EQU cannot.