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
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
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 devices 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.
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 users 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