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The Fortran Automatic Coding System for the IBM 704 (October 15, 1956), the first Programmer's Reference Manual for Fortran
The Fortran Automatic Coding System for the IBM 704 (October 15, 1956), the first Programmer's Reference Manual for Fortran
Paradigm: multi-paradigm: procedural, imperative, structured, object-oriented
Appeared in: 1957
Designed by: John W. Backus
Developer: John W. Backus & IBM
Typing discipline: strong, static
Major implementations: GFortran, Open Watcom, XL Fortran, many others
Influenced: ALGOL 58, PL/I

Fortran (previously FORTRAN[1]) is a general-purpose[2], procedural,[3] imperative programming language that is especially suited to numeric computation and scientific computing. Originally developed by IBM in the 1950s for scientific and engineering applications, Fortran came to dominate this area of programming early on and has been in continual use for over half a century in computationally intensive areas such as climate modeling[1], computational fluid dynamics (CFD), computational physics, and computational chemistry.

Fortran (an acronym derived from The IBM Mathematical Formula Translating System) encompasses a lineage of versions, each of which evolved to add extensions to the language while retaining compatibility with previous versions. Successive versions have added support for processing of character-based data (FORTRAN 77), array programming, module-based programming and object-based programming (Fortran 90/95), and object-oriented and generic programming (Fortran 2003).


[edit] History

An IBM 704 mainframe (image courtesy of LLNL)
An IBM 704 mainframe (image courtesy of LLNL)
FORTRAN code on a punch card, showing the specialized uses of columns 1-5, 6 and 73-80.
FORTRAN code on a punch card, showing the specialized uses of columns 1-5, 6 and 73-80.

In late 1953, John W. Backus submitted a proposal to his superiors at IBM to develop a more efficient alternative to assembly language for programming their IBM 704 mainframe computer. A draft specification for The IBM Mathematical Formula Translating System was completed by mid-1954. The first manual for FORTRAN appeared in October 1956, with the first FORTRAN compiler delivered in April 1957. This was an optimizing compiler, because customers were reluctant to use a high-level programming language unless its compiler could generate code whose performance was comparable to that of hand-coded assembly language.

While the community was skeptical this new method could possibly out-perform hand-coding, it reduced the amount of programming statements necessary to operate a machine by a factor of 20, and quickly gained acceptance. Said creator John Backus during a 1979 interview with Think, the IBM employee magazine, "Much of my work has come from being lazy. I didn't like writing programs, and so, when I was working on the IBM 701 (an early computer), writing programs for computing missile trajectories, I started work on a programming system to make it easier to write programs,"

The language was widely adopted by scientists for writing numerically intensive programs, which encouraged compiler writers to produce compilers that could generate faster and more efficient code. The inclusion of a complex number data type in the language made Fortran especially suited to technical applications such as electrical engineering.

By 1960, versions of FORTRAN were available for the IBM 709, 650, 1620, and 7090 computers. Significantly, the increasing popularity of FORTRAN spurred competing computer manufacturers to provide FORTRAN compilers for their machines, so that by 1963 over 40 FORTRAN compilers existed. For these reasons, FORTRAN is considered to be the first widely used programming language supported across a variety of computer architectures.

The development of FORTRAN paralleled the early evolution of compiler technology; indeed many advances in the theory and design of compilers were specifically motivated by the need to generate efficient code for FORTRAN programs.

[edit] FORTRAN

The initial release of FORTRAN for the IBM 704 contained 32 statements, including:

  • DIMENSION and EQUIVALENCE statements
  • Assignment statements
  • Three-way arithmetic IF statement. [4]
  • IF statements for checking exceptions (ACCUMULATOR OVERFLOW, QUOTIENT OVERFLOW, and DIVIDE CHECK); and IF statements for manipulating sense switches and sense lights
  • GOTO, computed GOTO, ASSIGN, and assigned GOTO
  • DO loops
  • FREQUENCY statement (for providing optimization hints to the compiler) [5]


IBM's FORTRAN II appeared in 1958. The main enhancement was to support procedural programming by allowing user-written subroutines and functions. Six new statements were introduced:


Over the next few years, FORTRAN II would also add support for the DOUBLE PRECISION and COMPLEX data types.


A FORTRAN coding form, formerly printed on paper and intended to be used by programmers to prepare programs for punching onto cards by card punch operators.  Now obsolete.
A FORTRAN coding form, formerly printed on paper and intended to be used by programmers to prepare programs for punching onto cards by card punch operators. Now obsolete.

IBM also developed a FORTRAN III in 1958 that allowed for inline assembler code among other features; however, this version was never released as a product. Like the 704 FORTRAN and FORTRAN II, FORTRAN III included machine-dependent features that made code written in it unportable from machine to machine. Early versions of FORTRAN provided by other vendors suffered from the same disadvantage.


Starting in 1961, as a result of customer demands, IBM began development of a FORTRAN IV that removed the machine-dependent features of FORTRAN II (such as READ INPUT TAPE), while adding new features such as a LOGICAL data type, logical Boolean expressions and the logical IF statement as an alternative to the arithmetic IF statement. FORTRAN IV was eventually released in 1962, first for the IBM 7030 ("Stretch") computer, followed by versions for the IBM 7090 and IBM 7094.

[edit] FORTRAN 66

Perhaps the most significant development in the early history of FORTRAN was the decision by the American Standards Association (now ANSI) to form a committee to develop an "American Standard Fortran." The resulting two standards, approved in March 1966, defined two languages, FORTRAN (based on FORTRAN IV, which had served as a de facto standard), and Basic FORTRAN (based on FORTRAN II, but stripped of its machine-dependent features). The FORTRAN defined by the first standard became known as FORTRAN 66 (although many continued to refer to it as FORTRAN IV, the language upon which the standard was largely based). FORTRAN 66 effectively became the first "industry-standard" version of FORTRAN. FORTRAN 66 included:

  • Main program, SUBROUTINE, FUNCTION, and BLOCK DATA program units
  • DATA statement for specifying initial values
  • Intrinsic and EXTERNAL (e.g., library) functions
  • Assignment statement
  • GOTO, assigned GOTO, and computed GOTO statements
  • Logical IF and arithmetic (three-way) IF statements
  • DO loops
  • READ, WRITE, BACKSPACE, REWIND, and ENDFILE statements for sequential I/O
  • FORMAT statement
  • CALL, RETURN, PAUSE, and STOP statements
  • Hollerith constants in DATA and FORMAT statements, and as actual arguments to procedures
  • Identifiers of up to six characters in length
  • Comment lines

[edit] FORTRAN 77

After the release of the FORTRAN 66 standard, compiler vendors introduced a number of extensions to "Standard Fortran", prompting ANSI in 1969 to begin work on revising the 1966 standard. Final drafts of this revised standard circulated in 1977, leading to formal approval of the new FORTRAN standard in April 1978. The new standard, known as FORTRAN 77, added a number of significant features to address many of the shortcomings of FORTRAN 66:

  • CHARACTER data type, with vastly expanded facilities for character input and output and processing of character-based data
  • IMPLICIT statement
  • Block IF statement, with optional ELSE and ELSE IF clauses, to provide improved language support for structured programming
  • OPEN, CLOSE, and INQUIRE statements for improved I/O capability
  • Direct-access file I/O
  • PARAMETER statement for specifying constants
  • SAVE statement for persistent local variables
  • Generic names for intrinsic functions

An important practical extension to FORTRAN 77 was the release of MIL-STD-1753 in 1978. This specification, developed by the U. S. Department of Defense, standardized a number of features implemented by most FORTRAN 77 compilers but not included in the ANSI FORTRAN 77 standard. These features would eventually be incorporated into the Fortran 90 standard.

The IEEE 1003.9 POSIX Standard, released in 1991, provided a simple means for Fortran-77 programmers to issue POSIX system calls. Over 100 calls were defined in the document - allowing access to POSIX-compatible process control, signal handling, file system control, device control, procedure pointing, and stream I/O in a portable manner.

The development of a revised standard to succeed FORTRAN 77 would be repeatedly delayed as the standardization process struggled to keep up with rapid changes in computing and programming practice. In the meantime, as the "Standard FORTRAN" for nearly fifteen years, FORTRAN 77 would become the historically most important dialect.

[edit] Fortran 90

The much delayed successor to FORTRAN 77, informally known as Fortran 90, was finally released as an ANSI Standard in 1992. This major revision added many new features to reflect the significant changes in programming practice that had evolved since the 1978 standard:

  • Free-form source input, also with lowercase Fortran keywords
  • Modules, to group related procedures and data together, and make them available to other program units, including the capability to limit the accessibility to only specific parts of the module.
  • RECURSIVE procedures
  • A vastly improved argument-passing mechanism, allowing interfaces to be checked at compile time
  • User-written interfaces for generic procedures
  • Operator overloading
  • Derived/abstract data types
  • New data type declaration syntax, to specify the data type and other attributes of variables
  • Ability to operate on arrays (or array sections) as a whole in expressions and assignment statements, thus greatly simplifying math and engineering computations. These features include whole, partial and masked array assignments (such as the WHERE statement for selective assignment), array-valued constants and expressions, user-defined array-valued functions and array constructors.
  • Dynamic memory allocation by means of the ALLOCATABLE attribute and the ALLOCATE and DEALLOCATE statements
  • POINTER attribute, pointer assignment, and NULLIFY statement to facilitate the creation and manipulation of dynamic data structures
  • CASE construct for multi-way selection
  • EXIT and CYCLE statements for "breaking out" of normal DO loop iterations in an orderly way
  • Identifiers up to 31 characters in length
  • Inline comments
  • Portable specification of numerical precision under the user's control
  • New and enhanced intrinsic procedures.

[edit] Fortran 95

Fortran 95 was a minor revision, mostly to resolve some outstanding issues from the Fortran 90 standard. Nevertheless, Fortran 95 also added a number of extensions, notably from the High Performance Fortran specification:

  • FOR ALL and nested WHERE constructs to aid vectorization
  • User-defined PURE and ELEMENTAL procedures

An important supplement to Fortran 95 was the ISO technical report TR-15581: Enhanced Data Type Facilities, informally known as the Allocatable TR. This specification defined enhanced use of ALLOCATABLE arrays, prior to the availability of fully Fortran 2003-compliant Fortran compilers. Such uses include ALLOCATABLE arrays as derived type components, in procedure dummy argument lists, and as function return values. (ALLOCATABLE arrays are preferable to POINTER-based arrays because ALLOCATABLE arrays are guaranteed by Fortran 95 to be deallocated automatically when they go out of scope, eliminating the possibility of memory leakage. In addition, aliasing is not an issue for optimization of array references, allowing compilers to generate faster code than in the case of pointers.)

Another important supplement to Fortran 95 was the ISO technical report TR-15580: Floating-point exception handling, informally known as the IEEE TR. This specification defined support for IEEE floating-point arithmetic and floating point exception handling.

[edit] Fortran 2003

The most recent standard, Fortran 2003, is a major revision introducing many new features. A comprehensive summary of the new features of Fortran 2003 is available at the ISO Fortran Working Group (WG5) official Web site, and may be downloaded directly as a PDF file or gzipped PostScript file.

From that article, the major enhancements for this revision include:

  • Derived type enhancements: parameterized derived types, improved control of accessibility, improved structure constructors, and finalizers.
  • Object oriented programming support: type extension and inheritance, polymorphism, dynamic type allocation, and type-bound procedures.
  • Data manipulation enhancements: allocatable components (incorporating IEEE TR 15581), deferred type parameters, VOLATILE attribute, explicit type specification in array constructors and allocate statements, pointer enhancements, extended initialization expressions, and enhanced intrinsic procedures.
  • Input/output enhancements: asynchronous transfer, stream access, user specified transfer operations for derived types, user specified control of rounding during format conversions, named constants for preconnected units, the FLUSH statement, regularization of keywords, and access to error messages.
  • Procedure pointers.
  • Support for IEEE floating-point arithmetic and floating point exception handling (incorporating IEEE TR 15580).
  • Interoperability with the C programming language.
  • Support for international usage: access to ISO 10646 4-byte characters and choice of decimal or comma in numeric formatted input/output.
  • Enhanced integration with the host operating system: access to command line arguments, environment variables, and processor error messages.

[edit] Fortran 2008

Efforts are underway to develop a revision to Fortran 2003, tentatively called Fortran 2008. As with Fortran 95, this is intended to be a minor upgrade, incorporating clarifications and corrections to Fortran 2003, as well as introducing a select few new capabilities. Proposed new capabilities include

A full list is in the report "The language features that have been chosen for Fortran 2008" PDF file.

[edit] The legacy of FORTRAN

Since Fortran has been around for nearly fifty years, there is a vast body of Fortran in daily use throughout the scientific and engineering communities. It is the primary language for some of the most intensive supercomputing tasks, such as weather and climate modeling, computational fluid dynamics, computational chemistry, quantum chromodynamics, simulations of long-term solar system dynamics, high-fidelity evolution artificial satellite orbits, and simulation of automobile crash dynamics. Indeed, one finds that even today, half a century later, floating-point benchmarks to gauge the performance of new computer processors are still written in Fortran (e.g., CFP2000, the floating-point component of the SPEC CPU2000 benchmarks), an eloquent tribute to the longstanding excellent floating-point performance of the language.

[edit] Language features

The Fortran language features described are intended to be a fairly comprehensive overview of the Fortran language; full details may be found in any of several Fortran textbooks. Only those features widely used in new programs are described, as few of the historic features are used in modern programs. Still, most have been retained in the language to maintain backward compatibility.

For more details on this topic, see Fortran language features.

[edit] Portability

Portability was a problem in the early days because there was no agreed standard—not even IBM's reference manual—and computer companies vied to differentiate their offerings from others by providing incompatible features. Standards have improved portability. The 1966 standard provided a reference syntax and semantics, but vendors continued to provide incompatible extensions. Although careful programmers were coming to realize that use of incompatible extensions caused expensive portability problems, and were therefore using programs such as The PFORT Verifier, it was not until after the 1977 standard, when the National Bureau of Standards (now NIST) published FIPS PUB 69, that processors purchased by the U.S. Government were required to diagnose extensions of the standard. Rather than offer two processors, essentially every compiler eventually had at least an option to diagnose extensions.

Incompatible extensions were not the only portability problem. For numerical calculations, it is important to take account of the characteristics of the arithmetic. This was addressed by Fox et. al. in the context of the 1966 standard by the PORT library. The ideas therein became widely used, and were eventually incorporated into the 1990 standard by way of intrinsic inquiry functions. The widespread (now almost universal) adoption of the IEEE 754 standard for binary floating-point arithmetic has essentially removed this problem.

Access to the computing environment (e.g. the program's command line, environment variables, textual explanation of error conditions) remained a problem until it was addressed by the 2003 standard.

Large collections of "library" software that could be described as being loosely-related to engineering and scientific calculations, such as graphics libraries, have been written in C, and therefore access to them presented a portability problem. This has been addressed by incorporation of C interoperability into the 2003 standard.

It is now possible (and relatively easy) to write an entirely portable program in Fortran, even without recourse to a preprocessor.

[edit] Variants of Fortran

[edit] Specific variants

Vendors of high-performance scientific computers (e.g., Burroughs, CDC, Cray, Honeywell, IBM, Texas Instruments, and UNIVAC) added extensions to Fortran to take advantage of special hardware features such as instruction cache, CPU pipelines, and vector arrays. For example, one of IBM's FORTRAN compilers (H Extended IUP) had a level of optimization which reordered the machine language instructions to keep multiple internal arithmetic units busy simultaneously. Another example is CFD, a special variant of Fortran designed specifically for the ILLIAC IV supercomputer, running at NASA's Ames Research Center.

Object-Oriented Fortran was an object-oriented extension of Fortran, in which data items can be grouped into objects, which can be instantiated and executed in parallel. It was available for Sun, Iris, iPSC, and nCUBE, but is no longer supported.

Such machine-specific extensions have either disappeared over time or have had elements incorporated into the main standards; the major remaining extension is OpenMP, which is a cross-platform extension for shared memory programming. One new extension, CoArray Fortran, is intended to support parallel programming.

[edit] Fortran-based languages

Prior to FORTRAN 77, a number of preprocessors were commonly used to provide a friendlier language, with the advantage that the preprocessed code could be compiled on any machine with a standard FORTRAN compiler. Popular preprocessors included FLECS, MORTRAN, Ratfor, and Ratfiv. (Ratfor and Ratfiv, for example, implemented a remarkably C-like language, outputting preprocessed code in standard FORTRAN 66.[6])

The Fortran-95 Standard includes an optional Part 3 which defines an optional conditional compilation capability. This capability is often referred to as "CoCo".

Many Fortran compilers have integrated subsets of the C preprocessor into their systems.

SIMSCRIPT is an application specific Fortran preprocessor for modeling and simulating large discrete systems.

F was designed to be a clean subset of Fortran 95 that attempted to remove the redundant, unstructured, and deprecated features of Fortran, such as the EQUIVALENCE statement.

[edit] Code examples

The sample programs can be compiled and run with any standard Fortran compiler (see the end of this article for lists of compilers). Most modern Fortran compilers expect a file with a .f or .for extension (for FORTRAN 66 or FORTRAN 77 fixed-form source, although the FORTRAN 66 dialect may have to be selected specifically with a command-line option) or .f90/.f95 extension (for Fortran 90/95 free-form source, respectively).

For more details on this topic, see Fortran code examples.

[edit] FORTRAN quotations

For a programming language with a half-century legacy, FORTRAN not surprisingly has accumulated its share of jokes and folklore.

[edit] Standard jokes

  • "GOD is REAL unless declared INTEGER."
  • Joke, circa 1980 (following the standardization of FORTRAN 77): "Q: What will the scientific programming language of the year 2000 look like? ... A: Nobody knows, but its name will be FORTRAN."
  • A good FORTRAN programmer can write FORTRAN code in any language.
  • Computer Science without FORTRAN and COBOL is like birthday cake without ketchup and mustard.

[edit] From the historical record

  • FORTRAN, the infantile disorder, by now nearly 20 years old, is hopelessly inadequate for whatever computer application you have in mind today: it is now too clumsy, too risky, and too expensive to use.Edsger Dijkstra, circa 1970.
  • "The sooner the world forgets that FORTRAN ever existed, the better." (imputed to Joseph Weizenbaum)
  • "95 percent of the people who programmed in the early years would never have done it without Fortran." - Ken Thompson, circa 2005 [7]
  • "The primary purpose of the DATA statement is to give names to constants; instead of referring to pi as 3.141592653589793 at every appearance, the variable PI can be given that value with a DATA statement and used instead of the longer form of the constant. This also simplifies modifying the program, should the value of pi change." —Early FORTRAN manual for Xerox Computers[8]
  • "Consistently separating words by spaces became a general custom about the tenth century A.D., and lasted until about 1957, when FORTRAN abandoned the practice."Sun FORTRAN Reference Manual
  • "People are very flexible and learn to adjust to strange surroundings — they can become accustomed to read Lisp and Fortran programs, for example."Art of PROLOG, Leon Sterling and Ehud Shapiro, MIT Press
  • "Warning: Go directly to Jail. Do not pass GO. Do not collect $200." - Easter egg in the SDS/Xerox Sigma 7 FORTRAN compiler, when the statement GO TO JAIL was encountered. The message is from a Chance card in Monopoly.

[edit] References in popular culture

  • Computer folklore has attributed, incorrectly, the loss of an early U.S.A. rocket to an error in a Fortran program. For example, Recall the first American space probe to Venus, reportedly lost because Fortran cannot recognize a missing comma in a DO statement ... Hoare, C. A. R.. Hints on Programming Language Design.  in (October 1973) Sigact/Sigplan Symposium on Principles of Programming Languages. , reprinted in Horowitz. Programming Languages, A Grand Tour, 3rd ed. . See

[edit] References

[edit] Textbooks

  • Chapman, Stephen J. (2003). Fortran 90/95 for Scientists and Engineers, 2nd ed., McGraw-Hill. ISBN 0-07-282575-8. 
  • Ellis, T. M. R.; Ivor R. Phillips, Thomas M. Lahey (1994). Fortran 90 Programming, 1st ed., Addison Wesley. ISBN 0-201-54446-6. 
  • McCracken, Daniel D. (1961). A Guide to Fortran Programming. Wiley. 
  • McCracken, Daniel D. (1965). A Guide to Fortran IV Programming. Wiley. 
  • Metcalf, Michael; John Reid, Malcolm Cohen (2004). Fortran 95/2003 Explained. Oxford University Press. ISBN 0-19-852693-8. 
  • Nyhoff, Larry; Sanford Leestma (1995). FORTRAN 77 for Engineers and Scientists with an Introduction to Fortran 90, 4th ed., Prentice Hall. ISBN 0-13-363003-X. 

[edit] "Core" language standards

  • ANSI X3.9-1966. USA Standard FORTRAN, informally known as FORTRAN 66, American National Standards Institute. 
  • ANSI X3.9-1978. American National Standard – Programming Language FORTRAN, also known as ISO 1539-1980, informally known as FORTRAN 77, American National Standards Institute. 
  • ANSI X3.198-1992 (R1997). American National Standard – Programming Language Fortran Extended, informally known as Fortran 90, American National Standards Institute. 
  • ISO/IEC 1539-1:1997. Information technology – Programming languages – Fortran – Part 1: Base language, informally known as Fortran 95. There are a further two parts to this standard. Part 1 has been formally adopted by ANSI. 
  • ISO/IEC 1539-1:2004. Information technology – Programming languages – Fortran – Part 1: Base language, informally known as Fortran 2003. 

[edit] Related standards

[edit] External links

[edit] History

[edit] Standards

[edit] Tutorials

[edit] References

[edit] Code repositories

[edit] Free (open-source) compilers

  • g77: "g77 is a free Fortran 77 integrated into the GNU Compiler Collection (GCC)." Note: GCC, as of version 4.0, now includes GFortran as the default Fortran compiler in lieu of g77.
  • g95: "The goal of the g95 project is to create a free, open source Fortran 95 compiler and runtime libraries." Note: Once an open, collaborative effort, the project has now reverted to the exclusive control of the original g95 project creator.
  • GFortran: "The GNU Fortran 95 compiler, part of GCC." Distributed as part of GCC as of GCC 4.0, GFortran is likely to become the de facto "standard" Fortran compiler (much as g77 was through GCC 3.4.6).
  • High Performance Computing for Mac OS X — Fortran compilers for Mac OS X.
  • Open Research Compiler for Itanium(TM) Processor Family
  • Open64 is a suite of optimizing compiler development tools for Intel Itanium(TM) systems running Linux. The Open64 project is the continuation of the SGI Pro64(TM)
  • Open Watcom: "A joint effort between SciTech Software Inc, Sybase and the Open Source development community to maintain and enhance the Watcom C/C++ and Fortran cross compilers and tools."

[edit] Free (not open-source) compilers

[edit] Commercial compilers

[edit] Free graphical libraries/GUI

[edit] Proprietary/non-free graphical libraries/GUI

  • DISLIN: "DISLIN is a high-level plotting library for displaying data as curves, polar plots, bar graphs, pie charts, 3D-color plots, surfaces, contours and maps." For DOS, Linux, Mac OS X, OpenVMS, UNIX, Windows. "Free for non-commercial use" (but not free).
  • SansGUI: Requires Microsoft Visual C/C++ 6.0+ or Compaq Visual Fortran 6.1+. For Windows.
  • Winteracter: "The Fortran 9x GUI Toolset." For Linux, Mac OS X, Windows.

[edit] Testing Frameworks

  • fUnit - a unit testing framework.

[edit] Miscellaneous

Wikibooks has a book on the topic of

[edit] Notes

  1. ^ The names of earlier versions of the language through FORTRAN 77 were conventionally spelled in all-caps (FORTRAN 77 was the version in which the use of lowercase letters in keywords was strictly nonstandard). The capitalization has been dropped in referring to newer versions beginning with Fortran 90. The official language standards now refer to the language as "Fortran." Because the capitalization (or lack thereof) of the word FORTRAN was never 100% consistent in actual usage, and because many hold impassioned beliefs on the issue, this article, rather than attempt to be normative, adopts the convention of using the all-caps FORTRAN in referring to versions of FORTRAN through FORTRAN 77 and the title-caps Fortran in referring to versions of Fortran from Fortran 90 onward. This convention is reflected in the capitalization of FORTRAN in the ANSI X3.9-1966 (FORTRAN 66) and ANSI X3.9-1978 (FORTRAN 77) standards and the title caps Fortran in the ANSI X3.198-1992 (Fortran 90) standard.
  2. ^ Since FORTRAN 77, which introduced the CHARACTER data type.
  3. ^ Since FORTRAN II (1958).
  4. ^ Note: It is commonly believed that this statement corresponded to a three-way branch instruction on the IBM 704. This is not true, the 704 branch instructions all contained only one destination address (e.g., TZE - Transfer AC Zero, TNZ - Transfer AC Not Zero, TPL - Transfer AC Plus, TMI - Transfer AC Minus). The machine (and its successors in the 700/7000 series) did have a three-way skip instruction (CAS - Compare AC with Storage), which was probably the origin of this belief, but using this instruction to implement the IF would consume 4 instruction words, require the constant Zero in a word of storage, and take 3 machine cycles to execute; using the Transfer instructions to implement the IF could be done in 1 to 3 instruction words, required no constants in storage, and take 1 to 3 machine cycles to execute. An optimizing compiler like FORTRAN would most likely select the more compact and usually faster Transfers instead of the Compare (use of Transfers also allowed the FREQUENCY statement to optimize IFs, which could not be done using the Compare). Also the Compare considered -0 and +0 to be different values while the Transfer Zero and Transfer Not Zero considered them to be the same.
  5. ^ The FREQUENCY statement in FORTRAN was used originally and optionally to give branch probabilities for the three branch cases of the Arithmetic IF statement to bias the way code was generated and order of the basic blocks of code generated, in the global optimisation sense, were arranged in memory for optimality. The first FORTRAN compiler used this weighting to do a Monte Carlo simulation of the run-time generated code at compile time. It was very sophisticated for its time. This technique is documented in the original article in 1957 on the first FORTRAN compiler implementation by J. Backus, et al. Many years later, the FREQUENCY statement had no effect on the code, and was treated as a comment statement, since the compilers no longer did this kind of compile-time simulation. Below is a part of the 1957 paper, "The FORTRAN Automatic Coding System" by Backus, et al., with this snippet on the FREQUENCY statement and its use in a compile-time Monte Carlo simulation of the run-time to optimise the code generated. Quoting ...

    The fundamental unit of program is the basic block; a basic block is a stretch of program which has a single entry point and a single exit point. The purpose of section 4 is to prepare for section 5 a table of predecessors (PRED table) which enumerates the basic blocks and lists for every basic block each of the basic blocks which can be its immediate predecessor in flow, together with the absolute frequency of each such basic block link. This table is obtained by an actual "execution" of the program in Monte-Carlo fashion, in which the outcome of conditional transfers arising out of IF-type statements and computed GO TO'S is determined by a random number generator suitably weighted according to whatever FREQUENCY statements have been provided.

  6. ^ This is not altogether surprising, as Brian Kernighan, one of the co-creators of Ratfor, is also co-author of The C Programming Language.
  7. ^ Lohr, Steve. "John W. Backus, 82, Fortran Developer, Dies", 2007-03-19. Retrieved on 2007-03-20.
  8. ^ Prior to FORTRAN 77, which introduced the PARAMETER statement (the preferred method of defining constants), the DATA statement was the conventional way to define constants, although this statement merely specified initial values and did not prevent the corresponding variables from being modified (as by regular assignment statements).
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