Slide 5.12: Symbolic constants

Symbolic Constants

A symbolic constant is created by associating an identifier with either an integer expression or some text. Symbols are used only during the assembly. Code I is equal to Code II.

Code I

 .code
 TEMP = 100
 mov  eax, TEMP

	Symbol	Variable
Uses storage?	no	yes
Value changes at run time?	no	yes

Code II

 .code
  mov  eax, 100

Equal-Sign Directive
The equal-sign directive associates a symbol name with an integer expression. The syntax is:

         name  =  expression

When a program is assembled, all occurrences of name are replaced by expression, a 32-bit integer value, during the assembler's preprocessor step.

Why use symbols?

Easier to read, e.g., the variable TEMP versus the value 100.
Easier to maintain, e.g., changing the TEMP definition will affect all the values of TEMP occurrences.

Redefinitions

A symbol defined with ‘=’ can be redefined any number of times. The symbol changes value according to the sequential processing of your source code by the assembler.


 TEMP = 10
 mov  eax, TEMP     ; EAX = 10
 TEMP = 20
 mov  eax, TEMP     ; EAX = 20
 TEMP = 30
 mov  eax, TEMP     ; EAX = 30