The first chapter discusses the concept of macros: its definition, structure, usage, design goals, and the related prior work. This thesis principally concerns my work on OLMP (an On-Line Macro Processor for the Motorola 6800 Microprocessor), which is a macro processor which interacts with the user. It takes Motorola assembler source code and macro definitions as its input; after the appropriate editing and expansions, it outputs the expanded assembler source statements. The functional objectives, the design for implementation of OLMP, the basic macro format, and the macro definition construction are specified in Chapter Two. The software and the hardware environment of OLMP are discussed in the third chapter. The six modules of OLMP are the main spine of the fourth chapter. The comments on future improvement and how to link OLMP with the Motorola 6800 assembler are the major concern of the final chapter.