Execution  of  a  return  jump  instruction  causes  the address of the next instruction to be executed in the main program to be stored (usually in the entry cell of the subroutine). It then causes the instruction of the second cell of the subroutine to be executed. The last instruction to be executed will usually be a straight jump  to  the  address  contained  in  the  entry  cell.  Since a jump instruction specifies the address of the next instruction to be executed, the computer is provided with a means of returning to the main program once the subroutine has been executed. PROGRAM  CONSTRUCTION The process of writing a program is broken down into six basic steps. 1. Statement. A statement forms a clear compre- hensive  statement  of  the  problem. 2.  Analysis.  Analysis  consists  of  laying  out  the problem  in  a  form  that  will  lend  itself  to  arithmetical and/or   logical   analysis,   determining   what   logical decision must be made, and if data manipulation is required. 3. Flow diagram. A flow diagram, or chart, is an expansion of the steps in which special symbols are used  to  represent  the  various  operations  to  be performed and the sequence in which they are to fall. 4.  Encoding.  The  process  of  converting  the operations listed in the flow chart into language the computer will use, either machine instructions, words, or  compiler  statements. 5.  Debugging.  The  process  of  locating  errors  in the   program   is   called   “debugging.”   Various techniques are available for this purpose. A program may be written to include some aids for itself, or a separate debugging program may be run to test the operation of a malfunctioning program. For a simple program, a trial solution may be done on paper, and the computed results compared with those actually obtained at each step. 6.   Documentation. Documentation   is   very important because later changes may be warranted in a program, or it may be desirable to use subroutines from another program. Proper documentation will ensure  that  this  can  be  accomplished.  Documentation should include the following: l Program title . Problem statement l l l l l l l l Programmer’s   name Date Memory  area  used  and/or  number  of  cells  used Registers  used I/O devices required Flow diagram(s) Hard  copy  (program  listings,  especially  a listing  of  the  coded  instructions) Program  tapes FLOW  CHARTING The programmer constructs a program “map” in determining  a  solution  to  a  problem.  This  map  is commonly called a flow chart and serves a multitude of  important  functions. The  flow  chart  maps  the logical  steps  required,  decisions  to  be  reached,  and paths  to  be  followed  as  a  result  of  the  decisions. When  properly  annotated,  it  defines  input/output requirements,   address   allocations,   data   accuracy considerations,  and  register  usage.  A  flow  chart  is valuable  when  debugging  a  program  and  when making future changes. Flow charting can be constructed at various levels of  complexity.  A  high-level  flow  chart  consists  of  a few  symbols  and  presents  a  broad  overview  of  the problem.  A  low-level  flow  chart  may  approach  a one-to-one   correspondence   between   flow   chart symbol and program instruction. Usually, there will be  several  flow  charts  for  a  program  area.  These  may be  compared  to  the  prints  found  in  a  maintenance manual. These  prints  include  a  block  diagram  to show  the  relationship  of  major  units  (high-level), functional  block  diagrams  showing  the  major  circuits in a unit (intermediate-level), and the schematics of the circuits (low-level). Flow charts should beat such a   level   that   they   will   implement   all   the   uses previously   discussed. MAINTENANCE  PROGRAMS As  we  have  previously  stated,  a  routine  or program  is  a  series  of  instructions  that  control  the operations of a computer. Each instruction is used to cause some action that is part of the overall task the computer  must  perform. Therefore, an instruction may  be  considered  as  the  basic  building  block  of  a computer  program. 8-15