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