Computer Programming Languages – PL/1

One of my favorite computer languages is PL/1  or Programming Language / One.   This was IBM’s attempt to merge the scientific language of FORTRAN with the business language of COBOL and ALGOL to produce a single language which can operate equally in both spheres. 

I started using a derivative of PL/1 produced by Cornell university called ‘PL/C’ (I guess for Programming Language / Cornell).  PL/C used a subset of PL/1 and had the uncanny ability to always compile what was fed into it by taking a best guess at statements and producing ‘output’ statements for the rest.  This was at Bellingham High school where we would write out programs on punched cards to be taken up to the University mainframe for processing.  Later when I worked at the university as a ‘student programmer’ in High School I started using the real PL/1(F) compiler (as well a FORTRAN – never got that into COBOL at the time).

PL/1 is a procedural language as well as a structured language with procedures and functions and statements blocks and ‘do groups’ (think { … } in C).  Comments were in the /* COMMENTS  */ form like C today.

PL/1 supported a wide variety of data types.  Fixed binary, fixed decimal, float binary and even float decimal (which didn’t really exuist) and of course strings (fixed and varying (to a maximum) length).  Static, Automatic, or Based (think pointers) storage.  ?One of the interesting data types was the AREA – which is simply an area of memory and an area pointer – which is an offset pointer into that area.  There was also the TASK and EVENT data types which worked closely with miltitasking (read: multi-threading).

Yes, PL/1 supported a rather primitive multi-threading in the language itself.  The OS supported multitasking of course but usually one could only start a separate task via an assembly language.  PL/1 had a TASK option on the CALL statement which would invoke the subroutine as a separate task (sharing memory).  The EVENT datatype was a means of communicating between the tasks.  Along with the EVENT datatype there was the WAIT statements which would wait (or only check as I recall) for one or more events to complete.

PL/1 also supported the ‘PICTURE’ clause of COBOL as well as the scientific datatypes and functions of FORTRAN (even a COMPLEX datatype for complex numbers).  You could output data with a picture clause or Formatting statements like FORTRAN.

PL/1 also supported the various file types – sequential, random, indexed, etc…

PL/1 also included (and I think may have introduced) the concepts of exception handling.  This is where the program can set a trap to go off during specific exceptional conditions (like dividing by zero, a numeric overflow, or even the end of a page like this – to print the headings on the next page.:




PUT SKIP(2) EDIT(heading…)(…);


The ON ENDPAGE  specifies actions to take when a end-of-page condition is encountered on the SYSPRINT file (back then SYSPRINT was usually the line printer).  PUT PAGE SKIP(2) EDIT says to go to the next page, skip 2 lines and print…

PL/1 also supported automatic conversion. For example if you were to mathematically add a string to a number it would attempt to first convert the string to a number then do the add.  You could then assign the results to a string and PL/1 would convert the results to a numeric string.

PL/1 also had the PUT and GET DATA statement.  A curious statement which allows the user to specify the variable name and alue in the input (or have it shown in the output) like this:

A = 2, B = 2;

This was curious and I don’t think any other language has such a feature.  In fact if you specify the GET DATA(); without any variables  the user can specify any variable in the program.  I’ve only seen this used once – in a rather huge in-migration and out-migration modeling program I was looking once at written in PL/1.

What happened to PL/1?  I think PL/1 was simply ahead of its time and very complex.  PL/1 object programs were rather large compared with other languages at the time – and at this time having a low memory requirement was a premium.  It was also quite complex for the business or scientific user to grasp.

PL/1 is still available.  A company produces a PL/1 compiler for VMS (both VAX and Alpha).  PL/1(F) is also available via the Hercules 370 emulator and I believe MVS.  I believe Stratus also does most coding in PL/1.  I think IBM once had a ‘VisualAge PL/1’ which ran on windows.

Western Terminal System (see other post) also hosted a derivitive of PL/1 called WPL (Western Programming Language).  It was a very limited subset of PL/1.

Old Computers – Western Terminal System / Interdata 7/32


My first experience with a timeshare / terminal based system was on an Interdata 7/32 running a locally written (at Western Washington University) called Western Terminal System (or WTS).

I started using this in 1976 while a student at a local high school.  Before that my experience was with an IBM 360 over punched cards.


The Interdata 7/32 minicomputer allowed up to 1Mb of memory (these typically had 380-odd K – that’s less than a half a single Mb) and 32-bit words and 16 general registers (and I think 8 floating point registers).  One of the neat things about these was that they used an instruction set very close to the IBM 360 models.

In this case they connected from 16 to 20 character terminals up to a minicomputer with about 380K of memory.  Also attached were 4 200Mb disk drive cabinets (that’s a total of 800Mb).

In addition, as I recall the ‘blue’ system had a channel to channel interface with the IBM 360 mainframe.  Later on this allowed students to code IBM Jobs using the terminals and submit them to the IBM mainframe via this interface.  We thought that was kind of cool.

The simh project ( ) has Interdata simulators for the 7/32, 8/32, and their 16-bit minicomputers as well as an early version of UNIX (the interdata was the first non-DEC platform unix was ported to).  OS/32 (the Interdata/Concurrent OS for the 7/32 and 8/32 and 3200 series) is around.

The Interdata Company was purchased by Perkins-Elmer computers and eventually became part of Concurrent Computers which still exists.  There are still  a few 8/32’s running today in various locations).


The OS was a locally written timeshare system called ‘Western Terminal System’ (makes sense since it was at Western Washington University) consisting (and this is from memory) of a common low-level OS, basic meta-code interpreter, and basic (or WPL) to metadata translators.  It was used basically for CAI (Computer Assisted Instruction) using the Pilot language (all text based of course).  Users were given either a Private or Student userid and a user file.

Each CRT terminal had a 8K partition of memory and a ‘temp’ file of about 300 80 column lines of text (structured as 100 blocks of 3 80-column lines each).  Each user also had a user file of, on average, about 300 lines (also in 100 block of 3 80 character records) – the user file was ‘partitioned’ into seperate files sorta-kinda like a partitioned dataset – there was a directory and ‘data’ area.  And people used to have to compress their files often using a utility.  In addition all terminals shared a read-only ‘public’ file, file, which contains common utilities.

There were utilities to renumber basic files (fixing the gosubs correctly), list file contents, Edit files (line editing only) etc…

The basic language it hosted was a structured basic with do-blocks and the like.

In addition to basic there was PILOT used for authoring CAI courseware and WPL (Western Programming Language) a derivitive of PL/1.  Basic was somewhat integrated with pilot so you could define basic variables, do some simple statements, call basic subroutines.

A few of the terminals were Tektronix 1410 Storage tubes able to display graphics (but to erase anything you had to flash and erase the entire screen) – some of the routines and utilities were written to take advantage of the graphical nature.

Courseware was where you would sit at a terminal and enter the name of a course.  I remember taking their ‘FORBUS’ course which was an introduction to Fortran IV (and in fact that is how I learned Fortran).  They had a lot of other ‘corses’ which people can take, plus a couple of games.  Terminals were in one or two ‘terminal rooms’ which was staffed by student ‘consultants (sometimes called ‘insultants’).

One system, the ‘white’ system later was later given additional memory and a batch processor which included a Fortran compiler / interpreter.  The batch processor had (as I recall) 109K of memory.

I remember that they used to run SPSS in that small partitition.  The software (and batch processor) was modified so that certain fortran statements (I beleave it was the fortran REWIND 99 statement) would load in a new overlay of the SPSS program.  Output was to a queue or a hardcopy (mostly dot-matrix) printer – which were generally teletype printer terminals.  They were distinctive because they were noisy and when they executed a carrage return the printhead would slam (BANG!) into the side of the print area.  There were a few 1200 baud printers (I think they were DEC).

Later on Western installed a real VAX 780 running a version of VMS.  Soon after that my family moved away and I wasn’t able to access Western anymore.