Encyclopedia of Computer Science and Technology

456 stackstackOften a temporary storage data area is needed duringprocessing. For example, a program that calls a procedure(see procedures and functions) usually needs to passone or more data items to the procedure. These itemsare specified as arguments that will be matched to theprocedure’s defined parameters. For example, the procedurecallSquare (50, 50, 20)could draw a square whose upper left corner is at thescreen coordinates 50, 50 and whose length per side is 20pixels.When the compiler generates the machine code for thisstatement, that code will probably instruct the processorto store the numbers 50, 50, and 20 onto a stack. A stack issimply a list that represents successive locations in memoryinto which data can be inserted. The operation of astack can be visualized as being rather like the springloadedplatform onto which dishes are stacked for washingin some restaurants. As each dish (number) is added, thestack is “pushed.” Because only the item “on top” (the lastone added) can be removed (“popped”) at any given time,a stack is described as a LIFO (last in, first out) structure.(Note that this is different from a queue, where items canbe added or removed from either end [see queue].)Stacks are useful whenever nested items must betracked. For example, a procedure might call a procedurethat in turn calls another procedure. The stack can keeptrack of the parameters (as well as the calling address) foreach pending procedure.Stacks can also be used to evaluate nested arithmeticexpressions. For example, the expression that we write inconventional (prefix) notation as7 * 5 + 2can be represented internally in postfix form as:* + 5 7 2Here one stack can be used to hold the operators (* +) andone the operands (5 7 2). The evaluation then proceeds inthe following steps:Pop the * from the operator stackSince * is a binary operator (one that needstwo operands), pop the 5 and 7 from theoperand stackMultiply 5 and 7 to get 35.Pop the + from the operator stack.Pop the 35 (which is now on the top of theoperand stack) and the 2Add 35 and 2 to get 37.An interesting programming language uses this stackmechanism for all processing (see forth). In working withstacks, it may be necessary to keep in mind any limitationson the amount of memory allocated to the stack, although astack can also be implemented dynamically as a linked list(see list processing).Further Reading“Data Structures/Stacks and Queues.” Wikibooks. Availableonline. URL: http://en.wikibooks.org/wiki/Data_Structures/Stacks_and_Queues. Accessed August 22, 2007.Stallman, Richard(1953– )AmericanComputer ScientistRichard Stallman created superb software tools—the programsthat help programmers with their work. He wenton to spearhead the open source movement, a new way todevelop software.Stallman was born on March 16, 1953, in New YorkCity. He quickly showed prodigious talent for mathematicsand was exploring calculus by the age of eight. Not muchlater, his summer camp reading included a manual for theIBM 7094 mainframe belonging to one of the counselors.Fascinated with the idea of programming languages, youngRichard began writing simple programs, even though hehad no access to a computer.Fortunately, a high school honors program let himobtain some time on a mainframe, and his programmingtalents led to a summer job with IBM. While studyingfor his B.A. in physics at Harvard (which he received in1970), Stallman found himself sneaking across town tothe MIT Artificial Intelligence Lab. There he developedEmacs, a powerful text editor that could be programmedwith a language modeled after LISP, the favorite languageof AI researchers. While working on Emacs and othersystem software for the AI Lab, Stallman participatedin the unique MIT “hacker culture.” (During the 1970s,“hacker” still meant a creative computing virtuoso, not acyber-criminal.)Stallman’s experience in the freewheeling, competitiveyet cooperative atmosphere at MIT led him to decidein 1984 to start the Free Software Foundation, whichwould become his life’s work. Stallman and his colleaguesat the FSF worked through the 1980s to develop GNU.At the time, UNIX, the operating system of choice formost campuses and researchers, required an expensivelicense from Bell Laboratories. GNU (a recursive acronymfor “GNU’s Not UNIX”) was intended to include all thefunctionality of UNIX but with code that owed nothingto Bell Labs. Stallman’s key contributions to the projectincluded the GNU C compiler and debugger, as well ashis management of a cooperative effort in which manytalented programmers would coordinate their efforts overthe Internet.By the early 1990s, most of GNU was complete exceptfor a key component: the kernel containing the essentialfunctions of the operating system. A Finnish programmer