GetWiki

{{Short description|Instructions to be executed by a computer}}{{for|the TV program|The Computer Programme{{!}}The Computer Programme}}File:JavaScript_code.png|thumb|Source code for a computer program written in the JavaScriptJavaScriptA computer program is a sequence or set{{efn|The Prolog language allows for a database of facts and rules to be entered in any order. However, a question about a database must be at the very end.}} of instructions in a programming language for a computer to execute. It is one component of software, which also includes documentation and other intangible components.WEB A computer program in its human-readable form is called source code. Source code needs another computer program to execute because computers can only execute their native machine instructions. Therefore, source code may be translated to machine instructions using a compiler written for the language. (Assembly language programs are translated using an assembler.) The resulting file is called an executable. Alternatively, source code may execute within an interpreter written for the language.BOOK , If the executable is requested for execution, then the operating system loads it into memory and starts a process.BOOK , The central processing unit will soon switch to this process so it can fetch, decode, and then execute each machine instruction.BOOK If the source code is requested for execution, then the operating system loads the corresponding interpreter into memory and starts a process. The interpreter then loads the source code into memory to translate and execute each statement. Running the source code is slower than running an executable.BOOK , {{efn|An executable has each machine instruction ready for the CPU.}} Moreover, the interpreter must be installed on the computer.

Example computer program

The "Hello, World!" program is used to illustrate a language's basic syntax. The syntax of the language BASIC (1964) was intentionally limited to make the language easy to learn.BOOK , For example, variables are not declared before being used.BOOK , Also, variables are automatically initialized to zero. Here is an example computer program, in Basic, to average a list of numbers:BOOK , 10 INPUT "How many numbers to average?", A20 FOR I = 1 TO A30 INPUT "Enter number:", B40 LET C = C + B50 NEXT I60 LET D = C/A70 PRINT "The average is", D80 ENDOnce the mechanics of basic computer programming are learned, more sophisticated and powerful languages are available to build large computer systems.BOOK ,

History

{{See also|Computer programming#History|Programmer#History|History of computing|History of programming languages|History of software}}Improvements in software development are the result of improvements in computer hardware. At each stage in hardware's history, the task of computer programming changed dramatically.

Analytical Engine

(File:Diagram for the computation of Bernoulli numbers.jpg|thumb|right|Lovelace's description from Note G)In 1837, Jacquard's loom inspired Charles Babbage to attempt to build the Analytical Engine.BOOK The names of the components of the calculating device were borrowed from the textile industry. In the textile industry, yarn was brought from the store to be milled. The device had a "store" which consisted of memory to hold 1,000 numbers of 50 decimal digits each.BOOK Ada Lovelace worked for Charles Babbage to create a description of the Analytical Engine (1843).{{citation

Universal Turing machine

(File:Universal Turing machine.svg|350px|right)In 1936, Alan Turing introduced the Universal Turing machine, a theoretical device that can model every computation.BOOK It is a finite-state machine that has an infinitely long read/write tape. The machine can move the tape back and forth, changing its contents as it performs an algorithm. The machine starts in the initial state, goes through a sequence of steps, and halts when it encounters the halt state.BOOK , All present-day computers are Turing complete.BOOK ,

ENIAC

(File:ENIAC-changing_a_tube.jpg|thumb|right|Glenn A. Beck changing a tube in ENIAC)The Electronic Numerical Integrator And Computer (ENIAC) was built between July 1943 and Fall 1945. It was a Turing complete, general-purpose computer that used 17,468 vacuum tubes to create the circuits. At its core, it was a series of Pascalines wired together.BOOK

Stored-program computers

Instead of plugging in cords and turning switches, a stored-program computer loads its instructions into memory just like it loads its data into memory.BOOK The IBM System/360 (1964) was a family of computers, each having the same instruction set architecture. The Model 20 was the smallest and least expensive. Customers could upgrade and retain the same application software.BOOK IBM planned for each model to be programmed using PL/1.BOOK , A committee was formed that included COBOL, Fortran and ALGOL programmers. The purpose was to develop a language that was comprehensive, easy to use, extendible, and would replace Cobol and Fortran. The result was a large and complex language that took a long time to compile.BOOK , File:Dg-nova3.jpg|thumb|Switches for manual input on a Data General NovaData General NovaComputers manufactured until the 1970s had front-panel switches for manual programming.BOOK , The computer program was written on paper for reference. An instruction was represented by a configuration of on/off settings. After setting the configuration, an execute button was pressed. This process was then repeated. Computer programs also were automatically inputted via paper tape, punched cards or magnetic-tape. After the medium was loaded, the starting address was set via switches, and the execute button was pressed.

Very Large Scale Integration

A major milestone in software development was the invention of the Very Large Scale Integration (VLSI) circuit (1964).WEB Robert Noyce, co-founder of Fairchild Semiconductor (1957) and Intel (1968), achieved a technological improvement to refine the production of field-effect transistors (1963).BOOK Originally, integrated circuit chips had their function set during manufacturing. During the 1960s, controlling the electrical flow migrated to programming a matrix of read-only memory (ROM). The matrix resembled a two-dimensional array of fuses. The process to embed instructions onto the matrix was to burn out the unneeded connections. There were so many connections, firmware programmers wrote a computer program on another chip to oversee the burning. The technology became known as Programmable ROM. In 1971, Intel installed the computer program onto the chip and named it the Intel 4004 microprocessor.WEB The terms microprocessor and central processing unit (CPU) are now used interchangeably. However, CPUs predate microprocessors. For example, the IBM System/360 (1964) had a CPU made from circuit boards containing discrete components on ceramic substrates.WEB

Sac State 8008

(File:Sacstate 8008.jpg|thumb|Artist's depiction of Sacramento State University's Intel 8008 microcomputer (1972))The Intel 4004 (1971) was a 4-bit microprocessor designed to run the Busicom calculator. Five months after its release, Intel released the Intel 8008, an 8-bit microprocessor. Bill Pentz led a team at Sacramento State to build the first microcomputer using the Intel 8008: the Sac State 8008 (1972).WEB

x86 series

File:IBM_PC-IMG_7271_(transparent).png|thumb|right|The original IBM Personal ComputerIBM Personal ComputerIn 1978, the modern software development environment began when Intel upgraded the Intel 8080 to the Intel 8086. Intel simplified the Intel 8086 to manufacture the cheaper Intel 8088.WEB

• Memory instructions to set and access numbers and strings in random-access memory.
• Integer arithmetic logic unit (ALU) instructions to perform the primary arithmetic operations on integers.
• Floating point ALU instructions to perform the primary arithmetic operations on real numbers.
• Call stack instructions to push and pop words needed to allocate memory and interface with functions.
• Single instruction, multiple data (SIMD) instructions{{efn|introduced in 1999}} to increase speed when multiple processors are available to perform the same algorithm on an array of data.

Changing programming environment

File:DEC VT100 terminal transparent.png|thumb|right|The DEC VT100 (1978) was a widely used computer terminalcomputer terminalVLSI circuits enabled the programming environment to advance from a computer terminal (until the 1990s) to a graphical user interface (GUI) computer. Computer terminals limited programmers to a single shell running in a command-line environment. During the 1970s, full-screen source code editing became possible through a text-based user interface. Regardless of the technology available, the goal is to program in a programming language.

Programming paradigms and languages

Programming language features exist to provide building blocks to be combined to express programming ideals.BOOK , Ideally, a programming language should:

• express ideas directly in the code.
• express independent ideas independently.
• express relationships among ideas directly in the code.
• combine ideas freely.
• combine ideas only where combinations make sense.
• express simple ideas simply.

The programming style of a programming language to provide these building blocks may be categorized into programming paradigms.BOOK , For example, different paradigms may differentiate:

• procedural languages, functional languages, and logical languages.
• different levels of data abstraction.
• different levels of class hierarchy.
• different levels of input datatypes, as in container types and generic programming.

Each of these programming styles has contributed to the synthesis of different programming languages.A programming language is a set of keywords, symbols, identifiers, and rules by which programmers can communicate instructions to the computer.BOOK , They follow a set of rules called a syntax.

• Keywords are reserved words to form declarations and statements.
• Symbols are characters to form operations, assignments, control flow, and delimiters.
• Identifiers are words created by programmers to form constants, variable names, structure names, and function names.
• Syntax Rules are defined in the Backus–Naur form.

Programming languages get their basis from formal languages.BOOK , The purpose of defining a solution in terms of its formal language is to generate an algorithm to solve the underlining problem. An algorithm is a sequence of simple instructions that solve a problem.BOOK ,

Generations of programming language

The evolution of programming language began when the EDSAC (1949) used the first stored computer program in its von Neumann architecture.BOOK

• The first generation of programming language is machine language.BOOK

, Machine language requires the programmer to enter instructions using instruction numbers called machine code. For example, the ADD operation on the PDP-11 has instruction number 24576.BOOK

• The second generation of programming language is assembly language. Assembly language allows the programmer to use mnemonic instructions instead of remembering instruction numbers. An assembler translates each assembly language mnemonic into its machine language number. For example, on the PDP-11, the operation 24576 can be referenced as ADD in the source code. The four basic arithmetic operations have assembly instructions like ADD, SUB, MUL, and DIV. Computers also have instructions like DW (Define Word) to reserve memory cells. Then the MOV instruction can copy integers between registers and memory.

• The third generation of programming language uses compilers and interpreters to execute computer programs. The distinguishing feature of a third generation language is its independence from particular hardware.BOOK

, Early languages include Fortran (1958), COBOL (1959), ALGOL (1960), and BASIC (1964). In 1973, the C programming language emerged as a high-level language that produced efficient machine language instructions.BOOK , Whereas third-generation languages historically generated many machine instructions for each statement,BOOK , C has statements that may generate a single machine instruction.{{efn|Operators like x++ will usually compile to a single instruction.}} Moreover, an optimizing compiler might overrule the programmer and produce fewer machine instructions than statements. Today, an entire paradigm of languages fill the imperative, third generation spectrum.

• The fourth generation of programming language emphasizes what output results are desired, rather than how programming statements should be constructed. Declarative languages attempt to limit side effects and allow programmers to write code with relatively few errors. One popular fourth generation language is called Structured Query Language (SQL). Database developers no longer need to process each database record one at a time. Also, a simple statement can generate output records without having to understand how they are retrieved.

Imperative languages

(File:Object-Oriented-Programming-Methods-And-Classes-with-Inheritance.png|thumb|A computer program written in an imperative language)Imperative languages specify a sequential algorithm using declarations, expressions, and statements:BOOK ,

• A declaration introduces a variable name to the computer program and assigns it to a datatypeBOOK

, – for example: var x: integer;

• An expression yields a value – for example: 2 + 2 yields 4
• A statement might assign an expression to a variable or use the value of a variable to alter the program's control flow – for example: x := 2 + 2; if x = 4 then do_something();

Fortran

FORTRAN (1958) was unveiled as "The IBM Mathematical FORmula TRANslating system." It was designed for scientific calculations, without string handling facilities. Along with declarations, expressions, and statements, it supported:

• arrays.
• subroutines.
• (For loop1957: FORTRAN|"do" loops).

It succeeded because:

• programming and debugging costs were below computer running costs.
• it was supported by IBM.
• applications at the time were scientific.BOOK

, However, non-IBM vendors also wrote Fortran compilers, but with a syntax that would likely fail IBM's compiler. The American National Standards Institute (ANSI) developed the first Fortran standard in 1966. In 1978, Fortran 77 became the standard until 1991. Fortran 90 supports:

• records.
• pointers to arrays.

COBOL

COBOL (1959) stands for "COmmon Business Oriented Language." Fortran manipulated symbols. It was soon realized that symbols did not need to be numbers, so strings were introduced.BOOK , The US Department of Defense influenced COBOL's development, with Grace Hopper being a major contributor. The statements were English-like and verbose. The goal was to design a language so managers could read the programs. However, the lack of structured statements hindered this goal.BOOK , COBOL's development was tightly controlled, so dialects did not emerge to require ANSI standards. As a consequence, it was not changed for 15 years until 1974. The 1990s version did make consequential changes, like object-oriented programming.

Algol

ALGOL (1960) stands for "ALGOrithmic Language." It had a profound influence on programming language design.BOOK , Emerging from a committee of European and American programming language experts, it used standard mathematical notation and had a readable, structured design. Algol was first to define its syntax using the Backus–Naur form. This led to syntax-directed compilers. It added features like:

• block structure, where variables were local to their block.
• arrays with variable bounds.
• "for" loops.
• functions.
• recursion.

Algol's direct descendants include Pascal, Modula-2, Ada, Delphi and Oberon on one branch. On another branch the descendants include C, C++ and Java.

Basic

BASIC (1964) stands for "Beginner's All-Purpose Symbolic Instruction Code." It was developed at Dartmouth College for all of their students to learn. If a student did not go on to a more powerful language, the student would still remember Basic. A Basic interpreter was installed in the microcomputers manufactured in the late 1970s. As the microcomputer industry grew, so did the language.Basic pioneered the interactive session. It offered operating system commands within its environment:

• The 'new' command created an empty slate.
• Statements evaluated immediately.
• Statements could be programmed by preceding them with a line number.
• The 'list' command displayed the program.
• The 'run' command executed the program.

However, the Basic syntax was too simple for large programs. Recent dialects added structure and object-oriented extensions. Microsoft's Visual Basic is still widely used and produces a graphical user interface.

C

C programming language (1973) got its name because the language BCPL was replaced with B, and AT&T Bell Labs called the next version "C." Its purpose was to write the UNIX operating system. C is a relatively small language, making it easy to write compilers. Its growth mirrored the hardware growth in the 1980s. Its growth also was because it has the facilities of assembly language, but uses a high-level syntax. It added advanced features like:

• inline assembler.
• arithmetic on pointers.
• pointers to functions.
• bit operations.
• freely combining complex operators.

(File:Computer-memory-map.png|thumb|right|Computer memory map)C allows the programmer to control which region of memory data is to be stored. Global variables and static variables require the fewest clock cycles to store. The stack is automatically used for the standard variable declarations. Heap memory is returned to a pointer variable from the malloc() function.

• The global and static data region is located just above the program region. (The program region is technically called the text region. It is where machine instructions are stored.)

, One region is called the initialized data segment, where variables declared with default values are stored. The other region is called the block started by segment, where variables declared without default values are stored.

, They provide an interface to the function.

• The stack region is a contiguous block of memory located near the top memory address.BOOK

• The heap region is located below the stack. It is populated from the bottom to the top. The operating system manages the heap using a heap pointer and a list of allocated memory blocks.BOOK

C++

In the 1970s, software engineers needed language support to break large projects down into modules.BOOK , One obvious feature was to decompose large projects physically into separate files. A less obvious feature was to decompose large projects logically into abstract data types. At the time, languages supported concrete (scalar) datatypes like integer numbers, floating-point numbers, and strings of characters. Abstract datatypes are structures of concrete datatypes, with a new name assigned. For example, a list of integers could be called integer_list.In object-oriented jargon, abstract datatypes are called classes. However, a class is only a definition; no memory is allocated. When memory is allocated to a class and bound to an identifier, it is called an object.BOOK , Object-oriented imperative languages developed by combining the need for classes and the need for safe functional programming.BOOK , A function, in an object-oriented language, is assigned to a class. An assigned function is then referred to as a method, member function, or operation. Object-oriented programming is executing operations on objects.BOOK , Object-oriented languages support a syntax to model subset/superset relationships. In set theory, an element of a subset inherits all the attributes contained in the superset. For example, a student is a person. Therefore, the set of students is a subset of the set of persons. As a result, students inherit all the attributes common to all persons. Additionally, students have unique attributes that other people do not have. Object-oriented languages model subset/superset relationships using inheritance.BOOK , Object-oriented programming became the dominant language paradigm by the late 1990s.C++ (1985) was originally called "C with Classes."BOOK , It was designed to expand C's capabilities by adding the object-oriented facilities of the language Simula.BOOK , An object-oriented module is composed of two files. The definitions file is called the header file. Here is a C++ header file for the GRADE class in a simple school application:// grade.h// ---

---

// Used to allow multiple source files to include// this header file without duplication errors.// ------------------------------------------

---

#ifndef GRADE_H

1. define GRADE_H

class GRADE {public:





};

1. endif

A constructor operation is a function with the same name as the class name.BOOK , It is executed when the calling operation executes the new statement.A module's other file is the source file. Here is a C++ source file for the GRADE class in a simple school application:// grade.cpp// -----

---

#include "grade.h"GRADE::GRADE( const char letter ){

}int GRADE::grade_numeric( const char letter ){ }Here is a C++ header file for the PERSON class in a simple school application:// person.h// ----

---

#ifndef PERSON_H

1. define PERSON_H

class PERSON {public: };

1. endif

Here is a C++ source file for the PERSON class in a simple school application:// person.cpp// ------

---

#include "person.h"PERSON::PERSON ( const char *name ){ }Here is a C++ header file for the STUDENT class in a simple school application:// student.h// -----

---

#ifndef STUDENT_H

1. define STUDENT_H

1. include "person.h"
2. include "grade.h"

// A STUDENT is a subset of PERSON.// ----------------------------

---

class STUDENT : public PERSON{public: };

1. endif

Here is a C++ source file for the STUDENT class in a simple school application:// student.cpp// -------

---

#include "student.h"

1. include "person.h"

STUDENT::STUDENT ( const char *name ): { Here is a driver program for demonstration:// student_dvr.cpp// -----------

---

#include

1. include "student.h"

int main( void ){

}int grade_numeric( char letter ){ }In the constructor, the function calloc() is used instead of malloc() because each memory cell will be set to zero.Here is a C programming language header file for the PERSON abstract datatype in a simple school application:/* person.h *//* -------- */

1. ifndef PERSON_H
2. define PERSON_H

typedef struct{ } PERSON;/* Constructor *//* ----------- */PERSON *person_new( char *name );

1. endif

Here is a C programming language source file for the PERSON abstract datatype in a simple school application:/* person.c *//* -------- */

1. include "person.h"

PERSON *person_new( char *name ){



}Here is a C programming language header file for the STUDENT abstract datatype in a simple school application:/* student.h *//* --------- */

1. ifndef STUDENT_H
2. define STUDENT_H

1. include "person.h"
2. include "grade.h"

typedef struct{

} STUDENT;/* Constructor *//* ----------- */STUDENT *student_new( char *name );

1. endif

Here is a C programming language source file for the STUDENT abstract datatype in a simple school application:/* student.c *//* --------- */

1. include "student.h"
2. include "person.h"

STUDENT *student_new( char *name ){



}Here is a driver program for demonstration:/* student_dvr.c *//* ------------- */

1. include
2. include "student.h"

int main( void ){

return 0;}Here is a makefile to compile everything:

1. makefile
2. ----

   ---

   all: student_dvr

clean: student_dvr: student_dvr.c grade.o student.o person.o grade.o: grade.c grade.h student.o: student.c student.h person.o: person.c person.h The formal strategy to build object-oriented objects is to:BOOK ,

• Identify the objects. Most likely these will be nouns.
• Identify each object's attributes. What helps to describe the object?
• Identify each object's actions. Most likely these will be verbs.
• Identify the relationships from object to object. Most likely these will be verbs.

For example:

• A person is a human identified by a name.
• A grade is an achievement identified by a letter.
• A student is a person who earns a grade.

Syntax and semantics

(File:Terminal and non-terminal symbols example.png|300px|thumb|right|Production rules consist of a set of terminals and non-terminals.)The syntax of a programming language is a list of production rules which form its grammar.BOOK , A programming language's grammar correctly places its declarations, expressions, and statements.BOOK , Complementing the syntax of a language are its semantics. The semantics describe the meanings attached to various syntactic constructs.BOOK , A syntactic construct may need a semantic description because a production rule may have an invalid interpretation.BOOK , Also, different languages might have the same syntax; however, their behaviors may be different.The syntax of a language is formally described by listing the production rules. Whereas the syntax of a natural language is extremely complicated, a subset of the English language can have this production rule listing:BOOK

1. a sentence is made up of a noun-phrase followed by a verb-phrase;
2. a noun-phrase is made up of an article followed by an adjective followed by a noun;
3. a verb-phrase is made up of a verb followed by a noun-phrase;
4. an article is 'the';
5. an adjective is 'big' or
6. an adjective is 'small';
7. a noun is 'cat' or
8. a noun is 'mouse';
9. a verb is 'eats';

The words in bold-face are known as "non-terminals". The words in 'single quotes' are known as "terminals".BOOK , From this production rule listing, complete sentences may be formed using a series of replacements.BOOK

• sentence
• noun-phrase verb-phrase
• article adjective noun verb-phrase
• the adjective noun verb-phrase
• the big noun verb-phrase
• the big cat verb-phrase
• the big cat verb noun-phrase
• the big cat eats noun-phrase
• the big cat eats article adjective noun
• the big cat eats the adjective noun
• the big cat eats the small noun
• the big cat eats the small mouse

However, another combination results in an invalid sentence:

• the small mouse eats the big cat

Therefore, a semantic is necessary to correctly describe the meaning of an eat activity.One production rule listing method is called the Backus–Naur form (BNF).BOOK

• ::= which translates to is made up of a[n] when a non-terminal is to its right. It translates to is when a terminal is to its right.
• | which translates to or.
• which surround non-terminals.

Using BNF, a subset of the English language can have this production rule listing: Using BNF, a signed-integer has the production rule listing:BOOK

Notice the recursive production rule: This allows for an infinite number of possibilities. Therefore, a semantic is necessary to describe a limitation of the number of digits.Notice the leading zero possibility in the production rules: Therefore, a semantic is necessary to describe that leading zeros need to be ignored.Two formal methods are available to describe semantics. They are denotational semantics and axiomatic semantics.BOOK ,

Software engineering and computer programming

File:Two women operating ENIAC (full resolution).jpg|thumb|right|Prior to programming languages, Betty Jennings and Fran Bilas programmed the ENIACENIACSoftware engineering is a variety of techniques to produce quality computer programs.BOOK , Computer programming is the process of writing or editing source code. In a formal environment, a systems analyst will gather information from managers about all the organization's processes to automate. This professional then prepares a detailed plan for the new or modified system.BOOK , The plan is analogous to an architect's blueprint.

Performance objectives

The systems analyst has the objective to deliver the right information to the right person at the right time.BOOK , The critical factors to achieve this objective are:

1. The quality of the output. Is the output useful for decision-making?
2. The accuracy of the output. Does it reflect the true situation?
3. The format of the output. Is the output easily understood?
4. The speed of the output. Time-sensitive information is important when communicating with the customer in real time.

Cost objectives

Achieving performance objectives should be balanced with all of the costs, including:BOOK ,

1. Development costs.
2. Uniqueness costs. A reusable system may be expensive. However, it might be preferred over a limited-use system.
3. Hardware costs.
4. Operating costs.

Applying a systems development process will mitigate the axiom: the later in the process an error is detected, the more expensive it is to correct.BOOK ,

Waterfall model

The waterfall model is an implementation of a systems development process.BOOK , As the waterfall label implies, the basic phases overlap each other:BOOK ,

1. The investigation phase is to understand the underlying problem.
2. The analysis phase is to understand the possible solutions.
3. The design phase is to plan the best solution.
4. The implementation phase is to program the best solution.
5. The maintenance phase lasts throughout the life of the system. Changes to the system after it is deployed may be necessary.BOOK

, Faults may exist, including specification faults, design faults, or coding faults. Improvements may be necessary. Adaption may be necessary to react to a changing environment.

Computer programmer

A computer programmer is a specialist responsible for writing or modifying the source code to implement the detailed plan. A programming team is likely to be needed because most systems are too large to be completed by a single programmer.BOOK , However, adding programmers to a project may not shorten the completion time. Instead, it may lower the quality of the system. To be effective, program modules need to be defined and distributed to team members. Also, team members must interact with one another in a meaningful and effective way.Computer programmers may be programming in the small: programming within a single module.BOOK , Chances are a module will execute modules located in other source code files. Therefore, computer programmers may be programming in the large: programming modules so they will effectively couple with each other. Programming-in-the-large includes contributing to the application programming interface (API).

Program modules

Modular programming is a technique to refine imperative language programs. Refined programs may reduce the software size, separate responsibilities, and thereby mitigate software aging. A program module is a sequence of statements that are bounded within a block and together identified by a name.BOOK , Modules have a function, context, and logic:BOOK ,

• The function of a module is what it does.
• The context of a module are the elements being performed upon.
• The logic of a module is how it performs the function.

The module's name should be derived first by its function, then by its context. Its logic should not be part of the name. For example, function compute_square_root( x ) or function compute_square_root_integer( i : integer ) are appropriate module names. However, function compute_square_root_by_division( x ) is not.The degree of interaction within a module is its level of cohesion. Cohesion is a judgment of the relationship between a module's name and its function. The degree of interaction between modules is the level of coupling.BOOK , Coupling is a judgement of the relationship between a module's context and the elements being performed upon.

Cohesion

The levels of cohesion from worst to best are:BOOK ,

• Coincidental Cohesion: A module has coincidental cohesion if it performs multiple functions, and the functions are completely unrelated. For example, function read_sales_record_print_next_line_convert_to_float(). Coincidental cohesion occurs in practice if management enforces silly rules. For example, "Every module will have between 35 and 50 executable statements."
• Logical Cohesion: A module has logical cohesion if it has available a series of functions, but only one of them is executed. For example, function perform_arithmetic( perform_addition, a, b ).
• Temporal Cohesion: A module has temporal cohesion if it performs functions related to time. One example, function initialize_variables_and_open_files(). Another example, stage_one(), stage_two(), ...
• Procedural Cohesion: A module has procedural cohesion if it performs multiple loosely related functions. For example, function read_part_number_update_employee_record().
• Communicational Cohesion: A module has communicational cohesion if it performs multiple closely related functions. For example, function read_part_number_update_sales_record().
• Informational Cohesion: A module has informational cohesion if it performs multiple functions, but each function has its own entry and exit points. Moreover, the functions share the same data structure. Object-oriented classes work at this level.
• Functional Cohesion: a module has functional cohesion if it achieves a single goal working only on local variables. Moreover, it may be reusable in other contexts.

Coupling

The levels of coupling from worst to best are:

• Content Coupling: A module has content coupling if it modifies a local variable of another function. COBOL used to do this with the alter verb.
• Common Coupling: A module has common coupling if it modifies a global variable.
• Control Coupling: A module has control coupling if another module can modify its control flow. For example, perform_arithmetic( perform_addition, a, b ). Instead, control should be on the makeup of the returned object.
• Stamp Coupling: A module has stamp coupling if an element of a data structure passed as a parameter is modified. Object-oriented classes work at this level.
• Data Coupling: A module has data coupling if all of its input parameters are needed and none of them are modified. Moreover, the result of the function is returned as a single object.

Data flow analysis

(File:Sandwich data flow diagram.pdf|thumb|A sample function-level data-flow diagram)Data flow analysis is a design method used to achieve modules of functional cohesion and data coupling.BOOK , The input to the method is a data-flow diagram. A data-flow diagram is a set of ovals representing modules. Each module's name is displayed inside its oval. Modules may be at the executable level or the function level.The diagram also has arrows connecting modules to each other. Arrows pointing into modules represent a set of inputs. Each module should have only one arrow pointing out from it to represent its single output object. (Optionally, an additional exception arrow points out.) A daisy chain of ovals will convey an entire algorithm. The input modules should start the diagram. The input modules should connect to the transform modules. The transform modules should connect to the output modules.BOOK ,

Functional categories

File:Operating system placement (software).svg|thumb|upright|A diagram showing that the user interacts with the application software. The application software interacts with the operating system, which interacts with the hardware.]]Computer programs may be categorized along functional lines. The main functional categories are application software and system software. System software includes the operating system, which couples computer hardware with application software. The purpose of the operating system is to provide an environment where application software executes in a convenient and efficient manner.BOOK , Both application software and system software execute utility programs. At the hardware level, a microcode program controls the circuits throughout the central processing unit.

Application software

Application software is the key to unlocking the potential of the computer system.BOOK , Enterprise application software bundles accounting, personnel, customer, and vendor applications. Examples include enterprise resource planning, customer relationship management, and supply chain management software.Enterprise applications may be developed in-house as a one-of-a-kind proprietary software.BOOK , Alternatively, they may be purchased as off-the-shelf software. Purchased software may be modified to provide custom software. If the application is customized, then either the company's resources are used or the resources are outsourced. Outsourced software development may be from the original software vendor or a third-party developer.BOOK , The potential advantages of in-house software are features and reports may be developed exactly to specification.BOOK , Management may also be involved in the development process and offer a level of control.BOOK , Management may decide to counteract a competitor's new initiative or implement a customer or vendor requirement.BOOK , A merger or acquisition may necessitate enterprise software changes. The potential disadvantages of in-house software are time and resource costs may be extensive. Furthermore, risks concerning features and performance may be looming.The potential advantages of off-the-shelf software are upfront costs are identifiable, the basic needs should be fulfilled, and its performance and reliability have a track record. The potential disadvantages of off-the-shelf software are it may have unnecessary features that confuse end users, it may lack features the enterprise needs, and the data flow may not match the enterprise's work processes.One approach to economically obtaining a customized enterprise application is through an application service provider.BOOK , Specialty companies provide hardware, custom software, and end-user support. They may speed the development of new applications because they possess skilled information system staff. The biggest advantage is it frees in-house resources from staffing and managing complex computer projects. Many application service providers target small, fast-growing companies with limited information system resources. On the other hand, larger companies with major systems will likely have their technical infrastructure in place. One risk is having to trust an external organization with sensitive information. Another risk is having to trust the provider's infrastructure reliability.

Operating system

{{See also|Operating system}}File:Concepts- Program vs. Process vs. Thread.jpg|thumb|Program vs. Process vs. Thread Scheduling, Preemption, Context SwitchingContext SwitchingAn operating system is the low-level software that supports a computer's basic functions, such as scheduling processes and controlling peripherals.In the 1950s, the programmer, who was also the operator, would write a program and run it. After the program finished executing, the output may have been printed, or it may have been punched onto paper tape or cards for later processing. More often than not the program did not work. The programmer then looked at the console lights and fiddled with the console switches. If less fortunate, a memory printout was made for further study. In the 1960s, programmers reduced the amount of wasted time by automating the operator's job. A program called an operating system was kept in the computer at all times.BOOK The term operating system may refer to two levels of software.BOOK

Kernel Program

(File:Kernel Layout.svg|thumb|A kernel connects the application software to the hardware of a computer.)The kernel's main purpose is to manage the limited resources of a computer:

• The kernel program should perform process scheduling,BOOK

(File:Virtual memory.svg|thumb|250px|Physical memory is scattered around RAM and the hard disk. Virtual memory is one continuous block.)

• The kernel program should perform memory management.

• The kernel program should perform file system management. The kernel has instructions to create, retrieve, update, and delete files.
• The kernel program should perform device management. The kernel provides programs to standardize and simplify the interface to the mouse, keyboard, disk drives, printers, and other devices. Moreover, the kernel should arbitrate access to a device if two processes request it at the same time.
• The kernel program should perform network management.BOOK

• The kernel program should provide system level functions for programmers to use.BOOK

• Programmers access files through a relatively simple interface that in turn executes a relatively complicated low-level I/O interface. The low-level interface includes file creation, file descriptors, file seeking, physical reading, and physical writing.
• Programmers create processes through a relatively simple interface that in turn executes a relatively complicated low-level interface.
• Programmers perform date/time arithmetic through a relatively simple interface that in turn executes a relatively complicated low-level time interface.BOOK

• The kernel program should provide a communication channel between executing processes.BOOK

Originally, operating systems were programmed in assembly; however, modern operating systems are typically written in higher-level languages like C, Objective-C, and Swift.{{efn|The UNIX operating system was written in C, macOS was written in Objective-C, and Swift replaced Objective-C.}}

Utility program

A utility program is designed to aid system administration and software execution. Operating systems execute hardware utility programs to check the status of disk drives, memory, speakers, and printers.BOOK , A utility program may optimize the placement of a file on a crowded disk. System utility programs monitor hardware and network performance. When a metric is outside an acceptable range, a trigger alert is generated.BOOK , Utility programs include compression programs so data files are stored on less disk space. Compressed programs also save time when data files are transmitted over the network. Utility programs can sort and merge data sets. Utility programs detect computer viruses.

Microcode program

(File:Not-gate-en.svg|thumb|96px|right|NOT gate)(File:NAND_ANSI_Labelled.svg|thumb|96px|right|NAND gate)(File:NOR_ANSI_Labelled.svg|thumb|96px|right|NOR gate)(File:AND_ANSI_Labelled.svg|thumb|96px|right|AND gate)(File:OR_ANSI_Labelled.svg|thumb|96px|right|OR gate)A microcode program is the bottom-level interpreter that controls the data path of software-driven computers.BOOK (Advances in hardware have migrated these operations to hardware execution circuits.) Microcode instructions allow the programmer to more easily implement the digital logic levelBOOK A logic gate is a tiny transistor that can return one of two signals: on or off.BOOK

• Having one transistor forms the NOT gate.
• Connecting two transistors in series forms the NAND gate.
• Connecting two transistors in parallel forms the NOR gate.
• Connecting a NOT gate to a NAND gate forms the AND gate.
• Connecting a NOT gate to a NOR gate forms the OR gate.

These five gates form the building blocks of binary algebra—the digital logic functions of the computer.Microcode instructions are mnemonics programmers may use to execute digital logic functions instead of forming them in binary algebra. They are stored in a central processing unit's (CPU) control store.BOOK These hardware-level instructions move data throughout the data path.The micro-instruction cycle begins when the microsequencer uses its microprogram counter to fetch the next machine instruction from random-access memory.BOOK The final step is to execute the instruction using the hardware module's set of gates.(File:ALU block.gif|thumb|right|A symbolic representation of an ALU)Instructions to perform arithmetic are passed through an arithmetic logic unit (ALU).BOOK Microcode instructions move data between the CPU and the memory controller. Memory controller microcode instructions manipulate two registers. The memory address register is used to access each memory cell's address. The memory data register is used to set and read each cell's contents.BOOK Microcode instructions move data between the CPU and the many computer buses. The disk controller bus writes to and reads from hard disk drives. Data is also moved between the CPU and other functional units via the peripheral component interconnect express bus.BOOK

Notes

{{Notelist}}

References

{{reflist|30em}}