Fortran (formerly FORTRAN, derived from “Formula Translation”) is a general-purpose, 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 continuous use for over half a century in computationally intensive areas such as numerical weather prediction, finite element analysis, computational fluid dynamics, computational physics, crystallography and computational chemistry. It is a popular language for high-performance computing and is used for programs that benchmark and rank the world’s fastest supercomputers.
Fortran encompasses a lineage of versions, each of which evolved to add extensions to the language while usually retaining compatibility with prior versions. Successive versions have added support for structured programming and processing of character-based data (FORTRAN 77), array programming, modular programming and generic programming (Fortran 90), high performance Fortran (Fortran 95), object-oriented programming (Fortran 2003) and concurrent programming (Fortran 2008).
The names of earlier versions of the language through FORTRAN 77 were conventionally spelled in all-capitals (FORTRAN 77 was the last version in which the use of lowercase letters in keywords was strictly non-standard). 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” rather than all-caps “FORTRAN”.
An IBM 704 mainframe computer
In late 1953, John W. Backus submitted a proposal to his superiors at IBM to develop a more practical alternative to assembly language for programming their IBM 704 mainframe computer. Backus’ historic FORTRAN team consisted of programmers Richard Goldberg, Sheldon F. Best, Harlan Herrick, Peter Sheridan, Roy Nutt, Robert Nelson, Irving Ziller, Lois Haibt, and David Sayre. Its concepts included easier entry of equations into a computer, an idea developed by J. Halcombe Laning and demonstrated in the Laning and Zierler system of 1952..
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 the first optimizing compiler, because customers were reluctant to use a high-level programming language unless its compiler could generate code with performance comparable to that of hand-coded assembly language.
While the community was skeptical that this new method could possibly outperform hand-coding, it reduced the number of programming statements necessary to operate a machine by a factor of 20, and quickly gained acceptance. John Backus said 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, 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, and many advances in the theory and design of compilers were specifically motivated by the need to generate efficient code for FORTRAN programs.
The initial release of FORTRAN for the IBM 704 contained 32 statements, including:
- DIMENSION and EQUIVALENCE statements
- Assignment statements
- Three-way arithmetic IF statement, which passed control to one of three locations in the program depending on whether the result of the arithmetic statement was negative, zero, or positive
- IF statements for checking exceptions (ACCUMULATOR OVERFLOW, QUOTIENT OVERFLOW, and DIVIDE CHECK); and IF statements for manipulating sense switches and sense lights
- GO TO, computed GO TO, ASSIGN, and assigned GO TO
- DO loops
- Formatted I/O: FORMAT, READ, READ INPUT TAPE, WRITE, WRITE OUTPUT TAPE, PRINT, and PUNCH
- Unformatted I/O: READ TAPE, READ DRUM, WRITE TAPE, and WRITE DRUM
- Other I/O: END FILE, REWIND, and BACKSPACE
- PAUSE, STOP, and CONTINUE
- FREQUENCY statement (for providing optimization hints to the compiler).
The arithmetic IF statement was reminiscent of (but not readily implementable by) a three-way comparison instruction (CAS – Compare Accumulator with Storage) available on the 704. The statement provided the only way to compare numbers – by testing their difference, with an attendant risk of overflow. This deficiency was later overcome by “logical” facilities introduced in FORTRAN IV.
The FREQUENCY statement was used originally (and optionally) to give branch probabilities for the three branch cases of the arithmetic IF statement. The first FORTRAN compiler used this weighting to perform at compile time a Monte Carlo simulation of the generated code, the results of which were used to optimize the placement of basic blocks in memory – a very sophisticated optimization for its time. The Monte Carlo technique is documented in Backus et al.’s paper on this original implementation, The FORTRAN Automatic Coding System:
The fundamental unit of program is the basic block; a basic block is a stretch of program which has one entry point and one 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 running the program once 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.
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. A similar fate has befallen compiler hints in several other programming languages; for example C’s register keyword.
The first FORTRAN compiler reported diagnostic information by halting the program when an error was found and outputting an error code on its console. That code could be looked up by the programmer in an error messages table in the operator’s manual, providing them with a brief description of the problem.
Fixed layout and punched cards
FORTRAN code on a punched card, showing the specialized uses of columns 1–5, 6 and 73–80
Further information: Computer programming in the punched card era
Before the development of disk files, text editors and terminals, programs were most often entered on a keypunch keyboard onto 80-column punched cards, one line to a card. The resulting deck of cards would be fed into a card reader to be compiled. Punched-card codes included no lower-case letters or many special characters, and special versions of the IBM 026 keypunch were offered that would correctly print the repurposed special characters used in Fortran.
Reflecting punched-card input practice, Fortran programs were originally written in a fixed-column format, with the first 72 columns read into twelve 36-bit words.
A letter “C” in column 1 caused the entire card to be treated as a comment and ignored by the compiler. Otherwise, the columns of the card were divided into four fields:
- 1 to 5 were the label field: a sequence of digits here was taken as a label for use in DO or control statements such as GO TO and IF, or to identify a FORMAT statement referred to in a WRITE or READ statement. Leading zeros are ignored and 0 is not a valid label number.
- 6 was a continuation field: a character other than a blank or a zero here caused the card to be taken as a continuation of the statement on the prior card. The continuation cards were usually numbered 1, 2, etc. and the starting card might therefore have zero in its continuation column – which is not a continuation of its preceding card.
- 7 to 72 served as the statement field.
- 73 to 80 were ignored (the IBM 704’s card reader only used 72 columns).
Columns 73 to 80 could therefore be used for identification information, such as punching a sequence number or text, which could be used to re-order cards if a stack of cards was dropped; though in practice this was reserved for stable, production programs. An IBM 519 could be used to copy a program deck and add sequence numbers. Some early compilers, e.g., the IBM 650’s, had additional restrictions due to limitations on their card readers. Keypunches could be programmed to tab to column 7 and skip out after column 72. Later compilers relaxed most fixed-format restrictions, and the requirement was eliminated in the Fortran 90 standard.
Within the statement field, whitespace characters (blanks) were ignored outside a text literal. This allowed omitting spaces between tokens for brevity or including spaces within identifiers for clarity. For example, AVG OF X was a valid identifier, equivalent to AVGOFX, and 101010DO101I=1,101 was a valid statement, equivalent to 10101 DO 101 I = 1, 101 because the zero in column 6 is treated as if it were a space (!), while 101010DO101I=1.101 was instead 10101 DO101I = 1.101, the assignment of 1.101 to a variable called DO101I. Note the slight visual difference between a comma and a period.
Hollerith strings, originally allowed only in FORMAT and DATA statements, were prefixed by a character count and the letter H (e.g., 26HTHIS IS ALPHANUMERIC DATA.), allowing blanks to be retained within the character string. Miscounts were a problem.
IBM’s FORTRAN II appeared in 1958. The main enhancement was to support procedural programming by allowing user-written subroutines and functions which returned values, with parameters passed by reference. The COMMON statement provided a way for subroutines to access common (or global) variables. Six new statements were introduced:
- SUBROUTINE, FUNCTION, and END
- CALL and RETURN
Over the next few years, FORTRAN II would also add support for the DOUBLE PRECISION and COMPLEX data types.
Early FORTRAN compilers supported no recursion in subroutines. Early computer architectures supported no concept of a stack, and when they did directly support subroutine calls, the return location was often stored in one fixed location adjacent to the subroutine code (e.g. the IBM 1130) or a specific machine register (IBM 360 et seq), which only allows recursion if a stack is maintained by software and the return address is stored on the stack before the call is made and restored after the call returns. Although not specified in Fortran 77, many F77 compilers supported recursion as an option, and the Burroughs mainframes, designed with recursion built-in, did so by default. It became a standard in Fortran 90 via the new keyword RECURSIVE
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