instructions provide the computer with the ability to make decisions based on the results of previously generated data. Control instructions send commands to devices not under direct control of the control unit, such as input and output units.    The address portion of the control instruction does not specify a location in memory, but is usually a coded group that specifies an action be required of a particular piece of equipment. In a single-address computer, where each instruction refers to only one address or operand, the instructions are normally taken from the memory in sequential order. If one instruction comes from a certain location, such as X, the next instruction is usually taken from location X + 1. However, the execution of a logic instruction may produce a result that dictates that the next instruction is to be taken from an address as specified in a portion of the logic instruction. For example, the logic instruction may initiate certain operations in the computer to determine if the content of a given register in the arithmetic section is negative. If the answer is yes, the location of the next instruction is specified in an address section of the logic instruction. If the answer is no, the next instruction would be taken from the next sequential location in the memory. Every computer provides circuitry for a variety of logic instructions, thus providing the capability of selecting alternate instruction sequences if certain desirable or undesirable conditions exist. The ability to branch at key points is the special feature of the computer that makes it able to perform such diverse tasks as missile control, accounting, and tactical air plotting. ARITHMETIC-LOGIC UNIT The arithmetic-logic unit (ALU) is the section in which arithmetic and logic operations are performed on the input or stored data. The operations performed in this unit include adding, subtracting, multiplying, dividing, counting, shifting, complementing, and comparing. Generally, information delivered to the control unit represents instructions, while information routed to the arithmetic unit represents data. Frequently, it is necessary to modify an instruction. This instruction may have been used in one form in one step of the program, but must be altered for a subsequent step. In such cases, the instruction is delivered to the arithmetic unit where it is altered by addition to or subtraction from another number in the accumulator. The resultant modified instruction is again stored in the memory unit for use later in the program. All arithmetic operations can be reduced to one of four processes: addition, subtraction, multiplication, or division. In most computers, multiplication involves a series of additions, and division is a series of subtractions. The arithmetic unit contains several registers. Each register is a unit that can store one “word” of computer data. These registers generally include the D, X, and Q register, and a unit called the accumulator (A register). The registers are so named for identification purposes only. During an arithmetic process, the D, X, and Q registers temporarily hold or store the numbers being used in the operation. These numbers are called the operands. The accumulator stores the result of the operation. The control unit instructs the arithmetic unit to perform the specified arithmetic operation as requested by the program. The control unit transfers the necessary information into the D, X, and Q registers from memory and controls the storage of the results in the accumulator or in some specific location in memory. The arithmetic unit also makes comparisons and produces yes or no or GO/NO-GO outputs as a result. The computer can be programmed so that ayes or GO result causes the computer to perform the next step in the program, while a no or NO-GO result causes the computer to jump several steps in the program. A computer can also be programmed so that a no result at a certain point in the program will cause the computer to stop and await instructions from a keyboard or other input device. INTERNAL DATA STORAGE UNIT In some digital computers, the internal data storage unit, or memory section, is constructed of small, magnetic cores, each capable of representing anon or off condition. An on condition is represented by a 1 and an off condition is represented by a 0. A system of these cores arranged in a matrix can store any computer word that is represented in binary form. All computers must contain facilities to store computer words or instructions until these instructions or words are needed in the performance of the computer calculations. Before the stored-program type of computer can begin to operate on its input data, it is first necessary to store, in memory, a sequence of instructions and all numbers 8-5