APL - Introduction

We now turn our attention to APL, a unique symbolic programming language that can be run on MTS.

APL, A Programming Language

The concepts behind APL came from work done by Kenneth E. Iverson at Harvard in the late 1950s. He wrote the book A Programming Language from which APL got its name. He moved to IBM in the early 1960s and helped produce the first working version of the language. IBM distributed versions of APL in the 1960s and 1970s, during which time the language was refined into APL2. Implementations were made for other architectures, including microcomputers in the 1980s.

APL is unique for its use of special symbols for functions and the ability to operate on multi-dimensional arrays. Put together, this allows a small amount of code to do a large amount of work. An example (from Wikipedia to compute the prime numbers from 1 to R:



The version of APL on MTS is based on APL\360, developed at IBM in the late 1960s. This was adapted to use the local MTS file system and devices, and portions for multi-user support were removed as they were not needed on MTS. Later versions of IBM APL did run on MTS but are not available on the D6 distribution due to copyright reasons.

APL symbols were supported using teletypewriters with a custom keyboard layout and typeballs that could display these symbols on paper.

APL Keyboard layout from Wikipedia. CC-SA 3.0

Not all users would have this special teletypwriter, so APL supports the standard keyboard and printer character set using transliterations for symbols. For example, the divide operator ÷ is replaced with / and the ceiling operator , which finds the maximum of its arguments, is replaced with either $MA or $CE.

The hardware used by MTS for APL is not supported on Hercules so we will need to use these transliterations when running MTS under emulation.


Unlike other languages seen so far, we do need to set up APL before using it by installing it from the D5 tapes. The below method was adapted from work done by user halfmeg on the H390-MTS list.

Start with a regular D6.0 setup as described in this guide. Ensure that MTS is not running before following these steps.

Get a copy of the D5 tapes from Bitsavers and extract into a temporary directory.

Locate the files d5.0t1.aws and d5.0t2.aws under the extraction directory and copy these to the Tapes directory under your MTS install

Edit your hercules.cnf and add these lines. These tape devices are unused in the stock D6.0 install; if you have already assigned these for your own use then change the device names here and in the instructions below.

# Add D5 tapes needed to restore APL
018B   3420   Tapes/d5.0t1.aws   ro  # T90B, D5.0T1  
018C   3420   Tapes/d5.0t2.aws   ro  # T90C, D5.0T2  

The batch instructions to restore APL from these disks is available as a card deck from my github repo on MTS languages. Download that file and copy it to Units/RDR1.txt under your Hercules install, replacing the existing file. Note that the whitespace in the first line is important, so clone the git repo or download the file as raw text.

Start up MTS as normal, including HASP. When it is running, type devinit c from the Hercules console to load the card deck. You should see the below printed on the MTS console if this worked.

00051 MTS **** Remove tape from T90B (6250 BPI)  
00051 MTS **** Remove tape from T90C (6250 BPI)  

The output from the batch job can be found on the printer in Hercules file Units/PTR2.txt. Examine it for any errors; you can ignore lines like You restored files saved before FEB. 22, 1988. You should see that job extracted files from the tape and set permissions appropriately.

Finally, test that it works by logging into a normal user account (eg ST01) and running

$run *APL,par=sp,noball

The APL start up message should appear. Type )LIB 1 and you should see this listing of library files:


Type )OFF to exit APL.

When you next shutdown MTS, you can comment out the two D5.0 tapes in hercules.cnf to free up these devices for future use.

Running a program using *APL

*APL is an interactive environment where you can enter expressions and program lines. To start APL, run *APL with the parameters sp (to print spaces after each operator) and noball (to indicate we are not using the special APL typeball.

APL prompts with six leading spaces. You can enter expressions and get results back immediately, aligned in column 1.

System commands start with ). )SOURCE will read lines from a given text file and execute them. )CONTINUE will save a copy of the current workspace to a binary file which will automatically be loaded next time you start APL. )OFF will exit APL.

Hello world

As an example, here's a simple program to print 'Hello, world!' five times. This uses a simple loop - there's probably a more concise way to do this.

First, create a file called hello.apl containing the following lines:

'Hello, world!'  
$GO 2 * N $LE 5

Then start APL and load the text file:

# $run *apl par=sp,noball
# Execution begins   16:30:56 
SAVED  16.30.28 05%27%17  

After you enter the )SOURCE command APL will read the file but will not prompt you it has completed. Press the ATTN key to interrupt APL and return control to you. You can then enter HELLO to run the loaded program:

Hello, world!  
Hello, world!  
Hello, world!  
Hello, world!  
Hello, world!  
  16.31.16 05%27%17 CONTINUE
     16.31.16 05%27%17
CONNECTED    0.00.19  
CPU TIME     0.00.00  
# Execution terminated   16:31:15  T=0.034 

In the next post we'll look at the APL language in more detail.

Further information

IBM's APL\360 Primer is a great first read as it introduces the APL\360 system and APL language in a tutorial form. The APL\360 User's Manual can then be consulted for more in-depth information.

A classic introduction to APL is "APL 360: An Interactive Approach" by Gillman and Rose. A copy can be found at the Software Preservation Group of the Computer History Museum.

UM Computing Center Memo 382 is a guide to the implementation of APL\360 on MTS. I recommend reading the printed copy of this memo from the above source as it includes the hand written APL symbols missing on the source copy.

PIL - Roman numerals

In the final part of this series, let's create a real program in PIL.

The problem

We will implement arabic to roman number conversion from Rosetta Code.

The algorithm we're going to use is similar to the one used there for BASIC:

  • Have a table of all distinct roman numbers ordered by size, including the -1 variants like IV. So roman(0) = "M", roman(1) = "CM", roman(2) = "D" etc.
  • Have another table with the same indices for their arabic equivalents. arabic(0) = 1000, arabic(1) = 900, arabic(2) = 500 etc.
  • Loop through each index. For each, if the input value is greater than the value of the arabic table at that value, accumulate the roman equivalent at the end of the output string and decrease the input value by the arabic amount. Keep doing this until the remaining input value is smaller than the arabic number.
  • So for input 2900 the steps would be
    • index 0, output -> "M", input -> 1900
    • index 0, output -> "MM" , input -> 900
    • index 1, output -> "MMCM", input -> 0 and end

The solution

As PIL is an interpreted language I'll show a lightly reformatted transcript of my session as I build up the program in separate parts (and make mistakes along the way). Let's get started!

# $run *pil
# Execution begins   20:09:19
  PIL/2: Ready

The tables

First we need to set up the tables for arabic numbers in part 1. I will use the number command so that PIL prompts me with line numbers followed by an underscore automatically.

*number 1, 0.01
&*1.0  _arabic(0) = 1000
&*1.01 _arabic(1) = 900
&*1.02 _arabic(2) = 500
&*1.03 _arabic(3) = 400
&*1.04 _arabic(4) = 100
&*1.05 _arabic(5) = 90
&*1.06 _arabic(6) = 50
&*1.07 _arabic(7) = 40
&*1.08 _arabic(8) = 10
&*1.09 _arabic(9) = 9
&*1.10 _arabic(10) = 5
&*1.11 _arabic(11) = 4
&*1.12 _arabic(12) = 1
&*1.13 _$unnumber

The unnumber command exits numbered line prompting mode. It needs to be prefixed with $ to be executed immediately rather than be entered as part of the program.

Let's run that immediately so we can check it looks correct

*do part 1
*type arabic
  arabic(0) =  1000.0
  arabic(1) =  900.0
  arabic(2) =  500.0
  arabic(3) =  400.0
  arabic(4) =  100.0
  arabic(5) =  90.0
  arabic(6) =  50.0
  arabic(7) =  40.0
  arabic(8) =  10.0
  arabic(9) =  9.0
  arabic(10) =  5.0
  arabic(11) =  4.0
  arabic(12) =  1.0

We can then do the same for the roman numbers.

*number 2, 0.01
&*2.0 _roman(0) = "M"
&*2.01 _roman(1) = "CM"
&*2.02 _roman(2) = "D"
&*2.03 _roman(3) = "CD"
&*2.04 _roman(4) = "C"
&*2.05 _roman(5) = "XC"
&*2.06 _roman(6) = "L"
&*2.07 _roman(7) = "XL"
&*2.08 _roman(8) = "X"
&*2.09 _roman(9) = "IX"
&*2.1 _roman(10) = "V"
&*2.11 _roman(11) = "IV"
&*2.12 _roman(12) = "I"
&*2.13 _$unnumber
*do part 2

The main loop

Let's now make the main loop to convert the number. We'll do it in three parts, first the loop over the indices. I put in some comments fir the function.

*number 5, 0.01
&*5.0 _* Main entry point to arabic -> roman converter
&*5.01 _* Input: a (arabic number to convert)
&*5.02 _* Output: r (roman number equivalent of a)
&*5.03 _for i = 0 to 12: do part 6
&*5.04 _done
&*5.05 _$unnumber

Next, the loop for each arabic number. We can use a for with a dummy variable and the while controlling how often it is run.

*number 6, 0.01
&*6.0 _for j = 0 while a >= arabic(i): do part 7
&*6.01 _done
&*6.02 _$unnumber

Finally, in part 7 build up the roman number string and decrease the arabic number.

*number 7, 0.01
&*7.0 _r = r + roman(i)
&*7.01 _a = a - arabic(i)
&*7.02 _done
&*7.03 _$unnumber

Let's see what these look like now.

*type part 5, part 6, part 7

  5.0    * Main entry point to arabic -> roman converter
  5.01   * Input: a (arabic number to convert)
  5.02   * Output: r (roman number equivalent of a)
  5.03   for i = 0 to 12: do part 6
  5.04   done

  6.0    for j = 0 while a >= arabic(i): do part 7
  6.01   done

  7.0    r = r + roman(i)
  7.01   a = a - arabic(i)
  7.02   done

Trying it out

We can set up the input number in a then call part 5 to convert. The output should go into r.

*a = 13
*do part 5
  Error at step 7.0: r = ?

Ah, r is not initialised so cannot be appended to. We can patch part 5 and try again.

*5.025 r = ""
*do part 5
*type r
  r = "XIII"
*type a
  a =  0.0

Great! There is a side effect though, the input value in a is wiped out as PIL does not have local variables.

Thinking about it, we are relying on the tables being initialised when we run part 5. We should really make it stand-alone by calling part 1 and 2 first.

*5.026 do part 1
*5.027 do part 2

Making it interactive

We should have a way to prompt for a number and then display the conversion.

*number 10, 0.01
&*10.0 _demand a
&*10.01 _do part 5
&*10.02 _type r
&*10.03 _$unnumber

*do part 10
& a = ? _1992
  r = "MCMXCII"

Unit tests!

It may be anachronistic, but we should have some unit tests to see if the conversion works. First let's define a unit test handler in part 20 that takes the arabic number in a, the expected result in rExpected and then checks this matches.

*number 20, 0.01
&*20.0 _do part 5
&*20.01 _if r = rExpected, then type "OK", r; else type "ERROR', r, rExpected
  Error at step 20.01: SYMBOLIC NAME TOO LONG
&*20.02 _if r = re, then type "OK", r; else type "ERROR", r, re
&*20.03 _done
&*20.04 _$unnumber

rExpected is too long for a variable number so we use a shorter name instead, re.

Let's test the tester out.

*re = "XLII"
*a = 42
*do part 20
  Error at step 20.01: SYMBOLIC NAME TOO LONG

Ah, the bad line is still there, so delete that and try again.

*delete step 20.01
*do part 20
  r = ""
  re = "XLII"

Wait, that's not right, why is the output in r blank?

 *type r
  r = ""
 *type a
  a =  0.0

Oh OK, a is clobbered. Let's set it up again.

*a = 42
*do part 5
*type r
  r = "XLII"
*do step 20.02
  r = "XLII"
*do step 20.02
  r = "XLII"
*type part 20

  20.0    do part 5
  20.02   if r = re, then type "OK", r; else type "ERROR", r, re
  20.03   done

*a = 42
*re = "XLII"
*do part 20
  r = "XLII"

That fixed it. Try the error case.

*a = 42
*re = "XXX"
*do part 20
  r = "XLII"
  re = "XXX"

With that done, set up the tests.

*number 21, 0.01
&*21.0 _a = 2009
&*21.01 _re = "MMIX"
&*21.02 _do part 20
&*21.03 _a = 1666
&*21.04 _re = "MDCLXVI"
&*21.05 _do part 20
&*21.06 _a = 3888
&*21.07 _re = "MMMDCCCLXXXVIII"
&*21.08 _do part 20
&*21.09 _done
&*21.1 _$unnumber

And run them.

*do part 21
  r = "MMIX"
  r = "MDCLXVI"

All green. However we did not test all cases such as zero, negative numbers, non-integral numbers etc.

Save and load

To confirm the program is all done and we are not relying on anything in the environment, save it to disk, quit and come back into PIL and try re-running.

*create "roman.pil"
*save as "roman.pil", all parts
# Execution terminated   18:51:16  T=0.279

# $run *pil
# Execution begins   18:51:37
  PIL/2: Ready
*load "roman.pil"
*do part 10
& a = ?  _42
  r = "XLII"
*do part 21
  r = "MMIX"
  r = "MDCLXVI"

The complete listing

*type all parts

  1.0    arabic(0) = 1000
  1.01   arabic(1) = 900
  1.02   arabic(2) = 500
  1.03   arabic(3) = 400
  1.04   arabic(4) = 100
  1.05   arabic(5) = 90
  1.06   arabic(6) = 50
  1.07   arabic(7) = 40
  1.08   arabic(8) = 10
  1.09   arabic(9) = 9
  1.1    arabic(10) = 5
  1.11   arabic(11) = 4
  1.12   arabic(12) = 1

  2.0    roman(0) = "M"
  2.01   roman(1) = "CM"
  2.02   roman(2) = "D"
  2.03   roman(3) = "CD"
  2.04   roman(4) = "C"
  2.05   roman(5) = "XC"
  2.06   roman(6) = "L"
  2.07   roman(7) = "XL"
  2.08   roman(8) = "X"
  2.09   roman(9) = "IX"
  2.1    roman(10) = "V"
  2.11   roman(11) = "IV"
  2.12   roman(12) = "I"

  5.0    * Main entry point to arabic -> roman converter
  5.01   * Input: a (arabic number to convert)
  5.02   * Output: r (roman number equivalent of a)
  5.025  r = ""
  5.026  do part 1
  5.027  do part 2
  5.03   for i = 0 to 12: do part 6
  5.04   done

  6.0    for j = 0 while a >= arabic(i): do part 7
  6.01   done

  7.0    r = r + roman(i)
  7.01   a = a - arabic(i)
  7.02   done

  10.0    demand a
  10.01   do part 5
  10.02   type r

  20.0    do part 5
  20.02   if r = re, then type "OK", r; else type "ERROR", r, re
  20.03   done

  21.0    a = 2009
  21.01   re = "MMIX"
  21.02   do part 20
  21.03   a = 1666
  21.04   re = "MDCLXVI"
  21.05   do part 20
  21.06   a = 3888
  21.07   re = "MMMDCCCLXXXVIII"
  21.08   do part 20
  21.09   done

Final thoughts

JOSS is a simple but well designed language - it's easy to pick up, has a carefully chosen set of features and does the job it's supposed to do well. Compared to BASIC it seems much more intuitive as a simple language for non-specialists who want to get numeric calculations done quickly. The lack of functions and local variables, plus the heavily interactive nature of the language makes it harder to write larger programs, but given the first version was running in 1963 it's quite an impressive feat of engineering.

PIL, the version of JOSS implemented on MTS, improves the usability of the original language, eg by not requiring a period at the end of each statement. There is enough integration with the operating system to make it usable. It would be interesting to know what type of use it got at UM.

Several languages were inspired by JOSS, including FOCAL on PDP-8s. It's also one of the influences on MUMPS, which is still in use today.

Further information

Full source code for this program can be found on github.

PIL - Language features

In the last post we saw some of the history of PIL and how to run it on MTS. We'll now take a closer look at the features of PIL. Examples shown can be entered directly into PIL after starting it with $run *pil.

Direct mode

PIL starts up in direct mode, where statements entered are immediately executed when you press RETURN. * is used by PIL as the prompt to enter input. You can use the TYPE statement and simple arithmetic expressions to make PIL act as a calculator:

* type 22 * 2
  22 * 2 =  44.0
* type 2 ** 16
  2 ** 16 =  65536.0

On MTS, PIL is case-insensitive for keywords and lines can optionally end with a period. Errors are immediately reported, usually starting with Eh?.

* TYPE 2+2.
  2+2 =  4.0

If the last character entered on the line is - then input will continue on the next line before it is executed. The prompt will change to & to show this continuation. * at the start of line can be used to make comments.

 * type 1 + 2 + 3 +-
 & 5 + 6
   1 + 2 + 3 +5 + 6 =  17.0
 * * comment here

Variables and types

Variables can be introduced by the optional keyword SET followed by a variable name and an assignment.

* set a = 2
* b = 3
* type a, b, a+b
  a =  2.0
  b =  3.0
  a+b =  5.0

PIL understands three types: numerical, which are stored as floating point values, character strings and Boolean values.

* a = 1 / 3
* b = "PIL"
* c = The True
* type a, b, c
  a =  0.3333333
  b = "PIL"
  c = The True

Booleans constants are 'The True' or 'The False' - something I've not seen in any other language. Strings can be up to 255 characters long and can be entered with single or double quotes as delimiters. Floats have 7 digits of precision and can be entered with exponential notation. Volume 12 mentions type 9.999999e64 as the maxomum value but on the version I'm running it seems 62 is the maximum exponent.

* type 9.999999e62
  9.999999e62 =  9.999999E+62
* type 9.999999e63

Variable names are up to 8 characters long, are case sensitive and are distinguished from keywords, as this silly example shows.

* set set = 1
* set SET = 2
* type set, SET
  set =  1.0
  SET =  2.0

Arrays are allowed with any number of dimensions, though once set the number of dimensions cannot be changed.

* x(1, -2) = 3
* x(3, 44.0) = 4
* type x
  x(1,-2) =  3.0
  x(3,44) =  4.0
* x(3,4,5) = 42
  Eh? ??-


Arithmetic expressions work mostly as expected. The absolute value can be taken by surrounding an expression with |; exponentiation is done with **. Functions for arithmetic operations such as square root, cosine, log etc generally have a short and long form and do not need parentheses unless needed to resolve ambiguities.

* type 1 + |-41|
  1 + |-41| =  42.0
* type the square root of 9
  the square root of 9 =  3.0
* type sqrt of 16
  sqrt of 16 =  4.0
* type sqrt of 25+1
  sqrt of 25+1 =  6.0
* type sqrt of (25+1)
  sqrt of (25+1) =  5.09902

There are functions for min/max, random numbers and extracting parts of a number, as well as special functions to get time (in 300ths of seconds since midnight), date, cpu/elapsed time and storage used.

* type the min of (1,2,3)
  the min of (1,2,3) =  1.0
* type the time, the date, the elapsed time, the cpu time
  the time =  1.963908E+07
  the date =  17133.0
  the elapsed time =  7.819461E+07
  the cpu time =  464.0
* type the total size, the size
  the total size =  188.0
  the size =  165.0

Boolean functions are similar to other languages, with # standing in for logical or.

* type 2 >= 3
  2 >= 3 = The False
* type 2 $lt 3
  2 $lt 3 = The True
* type the true # the false
  the true # the false = The True
* type the true & the false
  the true & the false = The False

Character expressions include length, case conversion, comparison and extraction.

* type the l of "abc"
  the l of "abc" =  3.0
* type the upper of "abC"
  the upper of "abC" = "ABC"
* type the first 3 characters of "abcde"
  the first 3 characters of "abcde" = "abc"
* type 2 $fc "abcde" + "X" + 1 $lc "abcde"
  2 $fc "abcde" + "X" + 1 $lc "abcde" = "abXe"
* type "aaa" > "AAA"
  "aaa" > "AAA" = The False

Finally, the type of an expression can be found and run time evaluation performed.

* type the mode of 42, the mode of the true, the mode of "abc"
  the mode of 42 =  1.0
  the mode of the true =  2.0
  the mode of "abc" =  3.0
* type the value of "2*21"
  the value of "2*21" =  42.0

Control flow

There's an IF statement with an optional ELSE clause. The IF and ELSE can be omitted but the punctuation is required.

* if 1 < 2, then type 'yes'; else type 'no'
* if 1 > 2, type 'yes'; type 'no'

For loops have a range or an increment and an optional clause to terminate.

* for i = 1 to 5: type i ** 2
  i ** 2 =  1.0
  i ** 2 =  4.0
  i ** 2 =  9.0
  i ** 2 =  16.0
  i ** 2 =  25.0
* for i = 1 by 2 to 10: type i ** 2
  i ** 2 =  1.0
  i ** 2 =  9.0
  i ** 2 =  25.0
  i ** 2 =  49.0
  i ** 2 =  81.0
* for i = 1 by 2 while i < 20: type i
  i =  1.0
  i =  3.0
  i =  5.0
  i =  7.0
  i =  9.0
  i =  11.0
  i =  13.0
  i =  15.0
  i =  17.0
  i =  19.0

Indirect mode

Lines entered that start with a number are treated as stored instructions, broken down by part and step, that can be run later. Here we define a program in part 1 consisting of four steps.

* 1.0 i = 5
* 1.1 type i
* 1.2 i = i * i
* 1.3 type i

This can then be run with DO, which will execute all steps in a part.

* do part 1
  i =  5.0
  i =  25.0

Variables set in a program remain after execution is completed and it is possible to run a single step at a time.

* type i
  i =  25.0
* do step 1.2
* do step 1.3
  i =  625.0

Steps can call other parts with DO: execution will resume after the part is finished. It's also possible to transfer execution with TO which will not return. DONE will return from the current part.

* type part 2, part 3

  2.0    i = 3
  2.1    do step 3
  2.2    i = 5
  2.3    do step 3
  2.4    i = 7
  2.5    to step 3
  2.6    type 'not reached'

  3.0    type i
  3.1    done
  3.2    type 'also not reached'

* do part 2
  i =  3.0
  i =  5.0
  i =  7.0

The above also shows it is possible to list out programs with TYPE PART. You can see all entered parts with TYPE ALL PARTS. With TYPE ALL STUFF you will see all variables and parts defined.

DELETE can be used to remove a step or a variable definition.

I/O and system access

We've seen TYPE used to display variables. You can prompt for a value to be entered with DEMAND

* delete part 1
* 1.0 demand i
* 1.1 i = i * 10
* 1.2 type i
* do part 1
 i = ?_ 3
  i =  30.0

There is also a formatting facility for input and output that is covered in the manual.

Programs can be saved to disk for future list with SAVE. This takes a file name (which must already exist) and what to save. For example:

save as 'x.pil', all stuff  

will save all parts and variables to x.pil. File format is plain text so it can be edited outside of PIL if needed. LOAD 'x.pil' will then load the file back into PIL.

The implementation of PIL on MTS includes access to system facilities such as file creation and device access. For example, CREATE 'x.pil' will create a new file.

In the next post we'll see how to construct a larger PIL program.

Further information

MTS volume 12 has a complete reference to PIL as implemented on MTS.

If you can find a copy of the book 'History of Programming Languages', edited by Richard Wexelblat, this has a great section about JOSS talking about its design principles.

PIL - Introduction

Screenshot of a teletypewriter running PIL, from 2 4 Using the Michigan Terminal System 6 01.

In this series of posts we'll look at the Pittsburgh Interpretative Language, or PIL, a simple interpreted language that can be used to do calculations and build small programs interactively.

The Pittsburgh Interpretative Language

PIL was developed at the University of Pittsburgh for the System/360 in the late 60s. It was based on one of the first interpreted languages JOSS that originated at the RAND Corporation in 1963. PIL improves on JOSS by providing improved debugging capabilities and error reporting.

The design goals for PIL were, according to MTS Volume 12:

PIL is oriented toward problem-solving, with program development and debugging facilities having highest priority. For the beginning user, PIL was designed to be clear, unambiguous, and hence, easily learned. For the experienced programmer, the language offers increased flexibility with statement structure and expanded capabilities for the solution of non-numeric problems. For the researcher, PIL reduces the amount of time and effort that must be expended in problem solving.

PIL can be used as a simple desktop calculator with variables

* set a = 3
* set b = 4
* type the square root of (a*a + b*b)
the square root of (a*a + b*b) =  5.0  

It can also be used to build simple programs interactively that can then be run:

* 1.01 demand a
* 1.02 demand b
* 1.03 type the square root of (a*a + b*b)
* do part 1
 a = ?_ 3
 b = ?_ 4
  the square root of (a*a + b*b) =  5.0


PIL on MTS is based on the second version of PIL, PIL/2. It was modified to integrate well with MTS files and system services.


No special installation instructions to get this language running - just do the standard D6.0 setup as described in this guide and then sign on as a regular user such as ST01.

Running a program using *PIL

Running the command *PIL on MTS will start the PIL interpreter. It is intended to be used in an interactive way where you enter commands and see the output directly. It can be used in batch mode by feeding commands into source but this is not the intended mode of operation. Inside the interpreter, programs and data can be loaded and saved with the load and save as statements.

Hello world

Let's see how to run a simple program to print 'Hello, world!' five times using PIL.

# $run *pil
# Execution begins   09:56:53 
  PIL/2: Ready
* for i = 1 to 5: type "Hello, world!"
  Hello, world!
  Hello, world!
  Hello, world!
  Hello, world!
  Hello, world!
* stop
# Execution terminated   09:59:06  T=0.004 

After starting PIL, enter the for statement at the * prompt. Output is shown immediately after the command is entered. Type stop to return to MTS.

In the next post we'll look at the language in more detail.

Further information

MTS volume 12 has a tutorial and reference for the PIL language and describes how it is integrated with MTS.

The video pictured at the top of this post, 2 4 Using the Michigan Terminal System 6 01, shows simple operation of PIL on a teletypewriter, starting from 21 minutes into the video.

The Pitt time-sharing system for the IBM system 360: Two year's experience. describes the operation of PIL at the University of Pittsburgh.

JOSS: Introduction To A Helpful Assistant is a very readable paper from the RAND Corporation on the language that inspired PIL.

The source code and implementation notes for PIL can be found in component 566 on tape 6.0T2 in the D6.0 distribution.

Source code for the hello world program can be found on github.

RATFOR & FLECS - Emirp primes

For the final post in this series, let's write a real program in RATFOR and FLECS and see how they compare with the original FORTRAN. We'll be implementing the reverse-primes emirp program we did before.

FLECS version

      IF (N .LE. 1) GOTO 200
      IF (N .EQ. 2 .OR. N .EQ. 3) GOTO 100
      IF (MOD(N,2) .EQ. 0) GOTO 200
      IF (MOD(N,3) .EQ. 0) GOTO 200
      DO (I=5,999999,2)
      IF (I*I .GT. N) GOTO 100
      IF (MOD(N,I) .EQ. 0) GOTO 200
 100  PRIME = .TRUE.
 200  PRIME = .FALSE.
      M = N
      R = 0
      UNTIL (M .LT. 1)
      R = R * 10
      R = R + MOD(M, 10)
      M = M / 10
      REVRSE = R
      R = REVRSE(N)
      IF (N .NE. R)
      IF (PRIME(N))
      IF (PRIME(R))
      EMIRP = .TRUE.
      EMIRP = .FALSE.
C     *** DISPLAY AN INTEGER ***  
      WRITE(6,50) N
 50   FORMAT(I10)
C     *** MAIN ENTRY POINT ***  
C     *** SHOW FIRST 20 EMIRPS ***  
      TO TEST-1
      N = 0
      I = 0
      WHILE (I .LT. 20)
      N = N + 1
      IF (EMIRP(N))
      CALL SHOW(N)
      I = I + 1
C     *** SHOW EMIRPS BETWEEN 7,700 AND 8,000 ***  
      TO TEST-2
      DO (N=7700,8000)
C     *** SHOW 10,000TH EMIRP ***  
      TO TEST-3
      N = 0
      DO (I=1,10000)
      REPEAT UNTIL (EMIRP(N)) N = N + 1
      CALL SHOW(N)

Apart from the FORMAT specification and the PRIME function we've eliminated all line numbers. PRIME could be written without line numbers but with the multiple paths out of the function that would need their own RETURN I think it's better this way.

The internal procedures come in handy, eliminating the need for subroutines for TEST1-3, though this does make N and I global which makes me a little uneasy if this was a larger program.

We use the block structure often, with UNTIL, WHILE and REPEAT ... UNTIL; this simplifies code, though without indentation it's a little hard to follow; the output of the preprocesor is useful here to show what it thinks the indentation should be, for example:

  86           TO TEST-1
  87           .  N = 0
  88           .  I = 0
  89           .  WHILE (I .LT. 20)
  90           .  .  N = N + 1
  91           .  .  IF (EMIRP(N))
  92           .  .  .  CALL SHOW(N)
  93           .  .  .  I = I + 1
  94           .  .  ...FIN
  95           .  ...FIN
  96           ...FIN

The compiler diagnostics also helped a lot with catching errors with missing FINs.


Now let's try writing the RATFOR version.

# Ratfor program to display emirps

######### Test if a number is prime #########
logical function prime(n)  
    integer n  # Number to test

    # Deal with numbers <= 3
    if (n < 1) goto 200
    if (n == 2 | n == 3) goto 100

    # Check if divisible by 2 or 3
    if (mod(n,2) == 0) goto 200
    if (mod(n,3) == 0) goto 200

    # See if divisible by 5, 7, ..., up to approx sqrt(n)
    for (i = 5; i < 1000000; i = i + 2) {
        if (I*I > n) goto 100
        if (mod(n,i) == 0) goto 200

 100  prime = .true.
 200  prime = .false.

######### Reverse an integer's digits #########
integer function revrse(n)  
    integer n  # Number to reverse
    integer m  # Copy of n from which we take digits
    integer r  # Reversed digits
    m = n
    r = 0
    while (m >= 1) {
        # Take last digit from m and append to r
        r = r * 10
        r = r + mod(m, 10)
        m = m / 10
    revrse = r

######### Test if an integer is an emirp #########
logical function emirp(n)  
    integer n       # Number to test
    integer revrse  # External function
    logical prime   # External function
    integer r       # Reversed digits of n
    r = revrse(n)
    emirp = .false.
    # n and r must both be prime and not the same value
    if (n .ne. r & prime(n) & prime(r)) {
        emirp = .true.

######### Display an integer #########
subroutine show(n)  
    integer n
    write(6,50) n
50  format(i10)  

######### Show first 20 emirps #########
subroutine test1  
    logical emirp   # External function
    integer i       # Count of emirps found
    integer n       # Number to test
    n = 0
    for (i = 1; i <= 20; i = i + 1) {
        repeat {
            n = n + 1
        } until (emirp(n))
        call show(n)

######### Show emirps between 7,700 and 8,000 #########
subroutine test2  
    logical emirp   # External function
    integer n       # Number to test
    for (n = 7700; n <= 8000; n = n + 1) {
        if (emirp(n)) {
            call show(n)

######### Show 10,000th emirp #########
subroutine test3  
    logical emirp   # External function
    integer i       # Count of emirps found
    integer n       # Number to test
    n = 0
    for (i = 1; i <= 10000; i = i + 1) {
        repeat {
            n = n + 1
        } until (emirp(n))
    call show(n)

######### Main entry point #########
call test1  
call test2  
call test3  

I feel right at home with the braces and the C style for loops, though I miss the increment operator ++. prime would be much better if I could just return (.true.) but that does not work on the version of RATFOR on MTS so we keep the line numbers and gotos.

With the above, plus the free form input (which was supported on MTS FORTRAN anyway) and the operators like < it was easy to write. However, I got precisely zero diagnostics from the RATFOR preprocessor, with all my typos caught by the FORTRAN compiler, from which I'd have to find the problem in the original source. Easy enough in a small program but would be painful in larger ones.

Final thoughts

RATFOR and FLECS both make writing FORTRAN easier and more pleasant at the cost of an extra step in the development process, and I found both succeed at that. RATFOR is clearer and easier to get started with (especially coming from a C background today); the implementation is almost aggressively simple, as the authors admit in their paper, and I wonder how well it would scale for writing larger programs. FLECS has a more robust implementation but a more diffuse design, such as two versions of switch; features like printing a neatly indented output would certainly help on MTS or its contemporaries but the language lacks the cosmetic features that make RATFOR easier to read.

Neither are much used today; FORTRAN 77 and beyond took some of these ideas and built them into the core language. The idea of translating a richer language into a widely used but less expressive language is still alive though: think of Coffeescript or Typescript producing Javascript.

Further information

Full source code for these programs can be found on github.

RATFOR & FLECS - Language Features

FORTRAN meeting From the UM Computing Center Newsletter, Volume 5 Number 14, 24 September 1975, via Google Books. Proposal 2) seems to indicate a different preprocessor was being considered for UM as well as FLECS, I wonder if this was RATFOR or something else?

Hi and welcome back. Today let's continue our exploration of RATFOR and FLECS by comparing the language features they add to vanilla FORTRAN. The quotes below are from the RATFOR paper and FLECS manual, links to which are provided at the end of this post. Code samples for FORTRAN and FLECS are shown in upper case, RATFOR in lower case.


RATFOR attempts to retain the merits of FORTRAN (universality, portability, efficiency) while hiding the worst FORTRAN inadequacies. The language is FORTRAN except for two aspects - [control flow and syntactic sugar] ... Throughout, the design principle which has determined what should be in RATFOR and what should not has been RATFOR doesn’t know any FORTRAN.

RATFOR focuses on control flow - if statements, blocks, looping - and cosmetics such as free form input, comments and other features that make FORTRAN more pleasant to write. By not knowing any FORTRAN, the design limits what features can be made available but also keeps it simple to implement and reduces the temptation to change FORTRAN into a different language altogether.

FLECS is a language extension of FORTRAN which has additional control mechanisms . These mechanisms make it easier to write FORTRAN by eliminating much of the clerical detail associated with constructing FORTRAN programs. FLECS is also easier to read and comprehend than FORTRAN.

FLECS also tries ti improve FORTRAN's control statements, taking ideas from several different languages including Pascal and Lisp. It has less cosmetic additions than RATFOR but adds the concept of internal procedures and includes features in the translator that help the programmer see the structure of their program.


RATFOR allows blocks of statements to be introduced within braces where FORTRAN would only allow a single statement. The fixed column format in classic FORTRAN is relaxed so any indentation is allowed. Multiple statements can appear on the same line if they are separated by semicolons.

if (x > 100) {  
   call error(x)
   err = 1; return

FLECS also has blocks which extend from the start of a control statement to the keyword FIN. It retains the fixed formatting of FORTRAN but prints a nicely indented view of the program when translating. So the example above would be entered as this in FLECS:

      IF (X .GT. 100)
      ERR = 1

and the translator would print

IF (X .GT. 100)  
.  ERR = 1

This is useful when entering programs via cards where it is difficult to get indentation right.

It's possible to have a single statement after a control structure in which case the FIN is not needed:

IF (X .GT. 100) CALL ERROR(X)  

RATFOR comments are introduced with # and apply from that point to the end of the line, less restrictive than C in FORTRAN and FLECS which must be in the first column.

% will stop RATFOR processing the rest of the line, passing it through to FORTRAN directly. FLECS will look for a FLECS statement in column 7 and if found will translate the line; if not found it will pass through the whole line to FORTRAN.

Textual substitution

RATFOR allows constants to be set with define SYMBOL VALUE; any use of SYMBOL in the RATFOR program will be replaced with VALUE in the generated FORTRAN program.

include FILE will insert a copy of FILE at that point in the program, just like C's #include.


RATFOR allows the now-familiar symbols <, <=, !=, | etc to be used instead of .LT., .LE., .NE., .OR. etc. FLECS retains the FORTRAN operators.


Text in RATFOR programs in single or double quotes is converted to FORTRAN nH strings. Backslash escapes the next character. FLECS keeps FORTRAN strings.


FORTRAN has a simple iF statement where only one statement can be executed if the condition is true. RATFOR extends this by allowing else and nested ifs. An else clause is attached to the nearest if.

if (x > 0) {  
  if (x > 10)
    write(6, 1) x
    write(6, 2) x
  weite(6, 3)

FLECS has IF and for negative tests UNLESS. It also has WHEN ... ELSE for a single positive and negative test.

The switch statement added in RATFOR looks like C but does not have break; the switch is exited after each case or default is executed. FLECS's equivalent is SELECT, so comparing the two:

switch (x) {  
  case 1: y=3
  case 2, 3: y=5
  default y=0
      SELECT (X)
      (1) Y=3
      (2) Y=5
      (3) Y=5
      (OTHERWISE) Y=0

FLECS has CONDITIONAL which looks a lot like LISP's cond:

      (X.LT.-5.0)  U = U+W
      (X.LE.1.0)   U = U+W+Z
      (X.LE.10.5)  U = U-Z
      (OTHERWISE)  U = 0


The FORTRAN DO loop has to have a line number marking the point where the loop will restart:

      DO 10 i = 1, n
      x(i) = 0.0
      y(i) = 0.0
      z(i) = 0.0

RATFOR replaces this with a block:

do i = 1, n {  
  x(i) = 0.0
  y(i) = 0.0
  z(i) = 0.0

It also allows break to exit a loop early and next to restart the loop like C's continue. It can be followed by an integer to say how many levels to apply, so break 2 would move out of a two level do statement immediately.

RATFOR also adds a while and for statement that look like C's - these allow immediate exit from the statement if the condition is true on entry, unlike in FORTRAN DO where the statement is always executed at least once (in the IBM implementation at least) and the conditional is tested at the end of the statement. A version of C's do ... while is provided as repeat ... until.

The FLECS equivalent for the above do loop would be:

      DO (I = 1, N)
      X(I) = 0.0
      Y(I) = 0.0
      Z(I) = 0.0

FLEC's WHILE construct is similar to RATFOR's, with the conditional tested before the loop starts. By using REPEAT WHILE the body of the loop is executed at least once and the test made at the end of the loop. UNTIL can be used instead of WHILE in both cases to indicate that the loop ends when the conditional becomes true

      X = 0
      UNTIL (X.EQ.5)
      X = X + 1


To return a value from a function in FORTRAN and FLECS you must assign a value to the name of the function:


In the RATFOR paper it sayd you can give return a value:

integer function decrement(i)  
integer i  
return (i-1)  

However, note this is not supported in the version supplied with MTS - it will just pass through such a return statement causing an error from the FORTRAN compiler.

Internal procedures

FLECS allows a group of statements to be defined as a procedure with TO which can then be called by giving its name. No parameters are passed - it uses global variables to communicate. The below example will print 5.

      X = 1
      WRITE(6,50) X
 50   FORMAT(I10) 
      TO INCREMENT-IT X = X + 1
      X = X * 2

Procedure names must include at least one hyphen and recursion is not allowed.


RATFOR runs as a simple translator, taking a RATFOR input file and producing a FORTRAN output file that must then be fed to the FORTRAN compiler. FLECS, as modified at UM, will both translate and call the FORTRAN compiler, producing machine code output that can be run directly.

Error handling

RATFOR will catch some errors, such as missing closing braces, but will otherwise delegate problems with the program to the FORTRAN compiler to catch, as it does not understand FORTRAN syntax. This could be difficult to trace back to the source of the error as the FORTRAN compiler would show the error in the generated FORTRAN, not the RATFOR original.

FLECS will find syntax errors and remove them from the program, allowing translation to continue at the cost of possibly causing further errors; it will not move on to compilation in this case.


Not surprisingly given its authors' roots, RATFOR was originally written in around 1000 lines of C using yacc. The authors say it took less than a week to implement. As C was not widely available in the mid 70's, a version of RATFOR in RATFOR was produced that would generate around 2500 lines of basic FORTRAN so it could be used anywhere.

The FLECS implementation comes in at around 2200 lines of FLECS and took around six months to develop according to comments in the source code.

Further information

See Kernighan's RATFOR paper or the FLECSUser's Manual (in component 673/22; I've uploaded a copy here) for more information on the languages.