Encyclopedia of Computer Science and Technology

asynchronous JavaScript and XML 29In this assembly language example, the “define byte” (.db) directive is used to assign one memory byte to each of the symbolic names (variables)firstnum, secondnum, and total. The two mov commands then load 2 and 3 into firstnum and secondnum, respectively. Firstnum isthen loaded into the processor’s accumulator (a), and secondnum is then added to it. Finally, the sum is moved into the memory locationlabeled total.allowing the programmer to use more human-friendlynames for instructions and data variables. The programthat translates between symbolic language and machinelanguage is the assembler.With a symbolic assembler, the programmer can givenames to data locations. Thus, instead of saying (and havingto remember) that the quantity Total will be in location&H100, the program can simply define a two-byte chunk ofmemory and call it Total:Total DBThe assembler will take care of assigning a physical memorylocation and, when instructed, retrieving or storing thedata in it.Most assemblers also have macro capability. This meansthat the programmer can write a set of instructions (a procedure)and give it a name. Whenever that name is used inthe program, the assembler will replace it with the actualcode for the procedure and plug in whatever variables arespecified as operands (see macro).ApplicationsIn the mainframe world of the 1950s, the development ofassembly languages represented an important first steptoward symbolic programming; higher-level languages suchas FORTRAN and COBOL were developed so that programmerscould express instructions in language that was morelike mathematics and English respectively. High-level languagesoffered greater ease of programming and sourcecode that was easier to understand (and thus to maintain).Gradually, assembly language was reserved for systems programmingand other situations where efficiency or the needto access some particular hardware capability required theexact specification of processing (see systems programmingand device driver).During the 1970s and early 1980s, the same evolutiontook place in microcomputing. The first microcomputerstypically had only a small amount of memory (perhaps8–64K), not enough to compile significant programs in ahigh-level language (with the partial exception of some versionsof BASIC). Applications such as graphics and games inparticular were written in assembly language for speed. Asavailable memory soared into the hundreds of kilobytes andthen megabytes, however, high level languages such as Cand C++ became practicable, and assembly language beganto be relegated to systems programming, including devicedrivers and other programs that had to interact directlywith the hardware.While many people learning programming today receivelittle or no exposure to assembly language, some understandingof this detailed level of programming is still usefulbecause it illustrates fundamentals of computer architectureand operation.Further ReadingAbel, Peter. IBM PC Assembly Language and Programming. 5th ed.Upper Saddle River, N.J.: Prentice Hall, 2001.Duntemann, Jeff. Assembly Language Step by Step: Programmingwith DOS and Linux. 2nd ed. New York: Wiley, 2000.Miller, Karen. An Assembly Language Introduction to ComputerArchitecture Using the Intel Pentium. New York: Oxford UniversityPress, 1999.asynchronous JavaScript and XML See ajax.