FORTRAN-20 is a FORTRAN compiler for TOPS-20. It supports the FORTRAN-77 standard (ANSI X3.9-1978 American National Standard Programming Language FORTRAN) with extensions and additions.

The purpose of this tutorial is to document the peculiarities of using FORTRAN-20 on TOPS-20 as opposed to other FORTRAN implementations on other operating systems; to highlight useful FORTRAN-20 extensions and additions that are not found in standard FORTRAN-77; and to briefly introduce the FORTRAN-77 programming language for those unfamiliar with it.

The order of presentation moves from more to less specialized and technical which runs counter to traditional organization, but the author feels this order is more convenient for the primary audience of programmers already familiar with FORTRAN but unfamiliar with the TOPS-20 operating system environment.

The FORTRAN compiler adds a report on CPU and elapsed time used by the program that prints at the end of every program run, like:

CPU time 0.08 Elapsed time 0.09

This message can be suppressed by adding the following line to your program source:

      CALL QUIETX

Although the TOPS-20 operating system as a whole survived the Y2K crisis, the FORTRAN DATE subroutine did not fare as well.

DATE is supposed to return a ten-character string representing the current system date with the format:

dd-mmm-yyb

Where:

However, since the year 2000, the DATE has set the yyb portion of the returned date to the first three digits of the current year, giving the same value for all years of a given decade. For example, for every year between 2010 and 2019, DATE returned “201” in the yyb portion of the date string.

This renders the year portion of DATE's output useless for most purposes. A work-around is to program a date subroutine in another language that correctly processes the TOPS-20 system date (such as MACRO assembly language) and call that subroutine from your FORTRAN program instead of the FORTRAN DATE subroutine.

FORTRAN uses logical device numbers for transmitting data to and from various input and output devices. In FORTRAN-20, some of these logical unit numbers were assigned to specific equipment that was common in the computer rooms of the 1970s and '80s but are not available in TWENEX.ORG's simulated PDP-10 environment.

When you are coding input/output statements, be sure to use a logical unit that is both valid and is associated with a device that is available on TWENEX.ORG.

The default logical unit (specified by “*” on READ and WRITE statements) varies according to the input/output statement:

Below is a list of FORTRAN-20 logical unit assignments. For file input or output, you may use any of the units assigned to disk (0, 1, 20-24, 30-99). If you do not specify a file name when you open the logical unit, FORTRAN-20 will use the default file name (in your currently connected directory) corresponding to the unit you are opening as shown in the table.

This section introduces potentially useful features that are unique to FORTRAN-20 and not included in the FORTRAN-77 standard.

(More to come: structured programming statements, in-line comments, …)

This section gives an brief overview of the FORTRAN-77 programming language for programmers with at least some experience with one or more other programming languages. It is not intended to be a comprehensive description or tutorial. Readers desiring a more complete introduction to FORTRAN are recommended to read the Fortran 77 Tutorial linked in the References section below.

(More to come: FORTRAN-77 cheat sheet, idiosyncracies (line layout, …).)

A sample program for the FORTRAN-20 compiler.

      program hello
      type *, ' Hello, World!'
      stop
      end                              

(Text on each line is preceded by six spaces.)

Digital Equipment Corporation. (1987, February). AA-N383B-TK FORTRAN Language Manual [Online]. Available: http://bitsavers.trailing-edge.com/pdf/dec/pdp10/TOPS20/fortran/AA-N383B-TK_FORTRAN_Language_Manual_Ver_10_May85.pdf.

P.H. Hargrove, S.T. Whitlock, E. Boman. (1997, December). Fortran 77 Tutorial [Online]. Available: http://www.stanford.edu/class/me200c/tutorial_77.