Define the PUBLIC and EXTRN directives- Assembler directives
PUBLIC and EXTRN directives are very significant to modular programming. PUBLIC used to declare that labels of data, code, or entire segments are available to other program modules. EXTRN (external) declares that labels are external to modules. Without these statements, modules couldn't be linked together to create a program by using modular programming techniques. They may link, however one module wouldn't be able to communicate to another.
PUBLIC directive is placed in opcode field of an assembly language statement to define a label as public so that label can be used by other modules.