Encyclopedia of Computer Science and Technology

array 23application suiteAn application suite is a set of programs designed to beused together and marketed as a single package. For example,a typical office suite might include word processing,spreadsheet, database, personal information manager, ande-mail programs.While an operating system such as Microsoft Windowsprovides basic capabilities to move text and graphics fromone application to another (such as by cutting and pasting),an application suite such as Microsoft Office makes it easierto, for example, launch a Web browser from a link within aword processing document or embed a spreadsheet in thedocument. In addition to this “interoperability,” an applicationsuite generally offers a consistent set of commands andfeatures across the different applications, speeding up thelearning process. The use of the applications in one packagefrom one vendor simplifies technical support and upgrading.(The development of comparable applications suitesfor Linux is likely to increase that operating system’s acceptanceon the desktop.)Applications suites have some potential disadvantagesas compared to buying a separate program for eachapplication. The user is not necessarily getting the bestprogram in each application area, and he or she is alsoforced to pay for functionality that may not be needed ordesired. Due to their size and complexity, software suitesmay not run well on older computers. Despite these problems,software suites sell very well and are ubiquitous intoday’s office.(For a growing challenge to the traditional standalonesoftware suite, see application service provider.)Further ReadingVillarosa, Joseph. “How Suite It Is: One-Stop Shopping for SoftwareCan Save You Both Time and Money.” Available online.Forbes magazine online. URL: http://www.forbes.com/buyers/070.htm. Accessed April 12, 2007.arithmetic logic unit (ALU)The arithmetic logic unit is the part of a computer systemthat actually performs calculations and logical comparisonson data. It is part of the central processing unit (CPU), andin practice there may be separate and multiple arithmeticand logic units (see cpu).The ALU works by first retrieving a code that representsthe operation to be performed (such as ADD). The code alsospecifies the location from which the data is to be retrievedand to which the results of the operation are to be stored.(For example, addition of the data from memory to a numberalready stored in a special accumulator register withinthe CPU, with the result to be stored back into the accumulator.)The operation code can also include a specificationof the format of the data to be used (such as fixed or floating-pointnumbers)—the operation and format are oftencombined into the same code.In addition to arithmetic operations, the ALU can alsocarry out logical comparisons, such as bitwise operationsthat compare corresponding bits in two data words, correspondingto Boolean operators such as AND, OR, and XOR(see bitwise operations and Boolean operators).The data or operand specified in the operation code isretrieved as words of memory that represent numeric data,or indirectly, character data (see memory, numeric data,and characters and strings). Once the operation is performed,the result is stored (typically in a register in theCPU). Special codes are also stored in registers to indicatecharacteristics of the result (such as whether it is positive,negative, or zero). Other special conditions called exceptionsindicate a problem with the processing. Commonexceptions include overflow, where the result fills more bitsthan are available in the register, loss of precision (becausethere isn’t room to store the necessary number of decimalplaces), or an attempt to divide by zero. Exceptions aretypically indicated by setting a flag in the machine statusregister (see flag).The Big PictureDetailed knowledge of the structure and operation of theALU is not needed by most programmers. Programmerswho need to directly control the manipulation of data inthe ALU and CPU write programs in assembly language(see assembler) that specify the sequence of operations tobe performed. Generally only the lowest-level operationsinvolving the physical interface to hardware devices requirethis level of detail (see device driver). Modern compilerscan produce optimized machine code that is almost as efficientas directly-coded assembler. However, understandingthe architecture of the ALU and CPU for a particular chipcan help predict its advantages or disadvantages for variouskinds of operations.Further ReadingKleitz, William. Digital and Microprocessor Fundamentals: Theoryand Applications. 4th ed. Upper Saddle River, N.J.: PrenticeHall, 2002.Stokes, Jon. “Understanding the Microprocessor.” Ars Technica.Available online. URL: http://arstechnica.com/paedia/c/cpu/part-1/cpu1-1.html. Accessed May 22, 2007.arrayAn array stores a group of similar data items in consecutiveorder. Each item is an element of the array, and it can beretrieved using a subscript that specifies the item’s locationrelative to the first item. Thus in the C language, the statementint Scores (10);sets up an array called Scores, consisting of 10 integer values.The statementScores [5] = 93;stores the value 93 in array element number 5. One subtlety,however, is that in languages such as C, the first element ofthe array is [0], so [5] represents not the fifth but the sixthelement in Scores. (Many version of BASIC allow for settingeither 0 or 1 as the first element of arrays.)