l Virtual (VOS) l Disk (DOS) Operating  systems  provide  the  link  between  the hardware and its user as well as enabling the execution of  operational  and/or  application  programs  designed  for specific  use. Application/Operational  Programs Programs for the computers you maintain will be used  in  tactical,  tactical  support,  and  nontactical platforms.  These  programs  are  designed  to  solve specific types of problems. They are commonly called application   programs,   operational   programs,   or processing programs. The programs used in tactical or tactical support platforms, such as CDS/NTDS or ASWOCs,  are  generally  called  operational  programs. The programs used with the SNAP systems (I and II) are, as a rule, called application programs. Programs available commercially that are designed to solve specific classes of problems are often called packaged software or off-the-shelf software. These include word processing,   database   management,   graphics, spreadsheet, and desktop publishing programs to name a few. Utility Programs Utility  programs  include  general  routines  or diagnostics   run   by   the   computer   to   test   other equipments or itself. A programmed operational and functional  appraisal  (POFA)  to  test  magnetic  tape  units and a diagnostic test for a computer are examples. Utilities can be run as stand-alone programs, such as microcomputer   diagnostics,   a   maintenance   test program (MTP), a POFA, and a standard test program (STP) using a standard test driver (STD). They can also be run as part of an operating system (if memory permits) or as online diagnostic tests such as on a SNAP system  or  NTDS. Utility programs also include programs  and  routines  to  perform  general  routine  tasks, such as disk/tape copy and print. These, too, can be stand-alone programs or they maybe included with the operating system or other programs. LEVELS OF INSTRUCTIONS The  CPU  executes  machine  instructions,  which manipulate the data within the functional units of the computer.  In  early  computers,  only  one  level  of machine instructions was used. In modern computers, this  only  remains  true  in  microprocessors  and  most microcomputers. For most computers, there are now two levels of machine instructions:  microinstruction and macroinstructions. In larger microprocessor- based devices (minicomputers and mainframes), each microinstruction  is  in  effect  a  predetermined  and installed  set  of  microinstruction. The particular device’s instruction set is made up of the highest levels (micro or macro) of machine instructions. The instruction set is the complete set of individual operations that can be executed or performed by  the  particular  microprocessor  or  computer.  In microprocessors, microcomputers, and microprocessor controlled  peripherals,  the  machine  instructions  are referred to as microinstruction, and the microprocessor executes them to perform the desired operations. In  mini  and  mainframe  computers,  the  machine instructions  are  actually  macroinstructions.  Once again, a microinstruction is a predetermined or preset sequence of microinstruction. Since most of the larger devices are microprocessor driven, it is necessary to break down the larger microinstruction into a series of smaller events that a microprocessor can handle. The microinstruction that make up the macroinstructions do not normally concern the computer programmer who uses   only   the   microinstruction   set. The microinstruction  are  usually  stored  in  some  form  of local memory, accessible only to the microprocessor translating and executing the macroinstructions. Instruction sets differ to some degree between computers, particularly between those of different manufacturers, types, and generations of computers. The actual number of instructions in an instruction set has a direct affect on the overall operation of the device. Computers with small instruction sets are easier to understand, and this simplifies both programming and maintenance. A large instruction set tends to support more specialized activities or functions that make the overall operation of the device more efficient or more tailored to the user’s requirements. An example of a large instruction set is one used on large mainframes aboard a ship. TYPES OF INSTRUCTIONS The flow of data in a computer is the result of instruction execution. Data can be exchanged between registers. It can be moved from one register to another. It can be moved from a register to a memory location or  vice  versa.  Arithmetic  instructions  can  be  performed using the contents of registers and memory locations. Logical instructions can be used to isolate bits in 8-3