Expression restrictions
Expressions can be categorized according to restrictions that apply to some of the assembler directives. One such example is the expression used in conditional statements like IF, where the expression must be evaluated at assembly time, and therefore cannot contain any external symbols.
The following expression restrictions are referred to in the description of each directive they apply to.
No forward
All symbols referred to in the expression must be known, no forward references are allowed.
No external
No external references in the expression are allowed.
Absolute
The expression must evaluate to an absolute value, a relocatable value (section offset) is not allowed.
Fixed
The expression must be fixed, which means that it must not depend on variable-sized instructions. A variable-sized instruction is an instruction that might vary in size depending on the numeric value of its operand.