Play with Docker and Pascal

Can you run 30-year-old Pascal code today? Yes! This guide shows you how to use Docker and the Free Pascal Compiler (FPC) to compile and execute your legacy Turbo Pascal programs.

December 1, 2023 · 4 min read

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Christophe

Markdown, WSL and Docker lover ~ Insatiable curious.

Cet article existe aussi en français — lire la version française.

This article is over a year old but was reviewed on July 30, 2026 — the content is still accurate.

Play with Docker and Pascal

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TL;DR

This article revives 30-year-old Turbo Pascal source code (pulled from the old SWAG archive) using the signumtemporis/fpc Docker image to compile .pas files into Windows executables with zero local install, running both a "Hello World" and an old school byte-to-binary conversion function written back in 1992.

Good old memories... During my studies (in 1991-1993), I was a huge fan of Turbo Pascal 7.0. It was the first language that I really learned, and I loved it so much. I used to spend dozens of hours behind my computer writing anything and everything.

If you still remember that time, Turbo Pascal 7.0 was used to create executables for MS-DOS.

The idea for this article came after the one written on assembly language: is it possible in 2023 to run Pascal code written 30 years earlier?

Turbo Pascal

The first thing to do was to find some old source code and to do that, I went back to the Sourceware Archive Group; known as SWAG.

The following parametrized query will retrieve some code I've published 30 years ago: https://www.google.com/search?q=avonture+site%3Ahttp%3A%2F%2Fwww.retroarchive.org. Out of nostalgia, I've also copied these sources here: https://github.com/cavo789/swag.

Hello world

Please run mkdir C:\tmp\pascal && cd C:\tmp\pascal in a MS-DOS console to create a folder called pascal and jump in it.

Please create a new file called Hello.pas with this content:

begin
    writeln('Hello world! I''m a Turbo Pascal source code');
end.

This is a complete Turbo Pascal program that prints a single line of text to the console. The `begin` and `end.` keywords mark the start and finish of the main program block.

As you know, Pascal is a compiled language and should then be compiled into an .exe. The command below will do this:

Powershell

$ docker run -it --rm -v %CD%:/app -w /app signumtemporis/fpc:cross.x86_64-win64.slim Hello.pas
Free Pascal Compiler version 3.2.2 [2021/12/10] for x86_64
Copyright (c) 1993-2021 by Florian Klaempfl and others
Target OS: Win64 for x64
Compiling Hello.pas
Linking Hello.exe
2 lines compiled, 0.1 sec, 32432 bytes code, 1508 bytes data
▋

Our executable has been created. Time to start it by running Hello.exe:

Powershell

$ Hello.exe
Hello world! I'm a Turbo Pascal source code▋

Voilà, we've successfully created our first Pascal code in 2023.

This blog has a small collection of such "run language X without installing it" articles: Java, Python and PHP.

Docker CLI reminder

As a reminder, the used Docker run commands are (almost always the same):

-it to start Docker interactively, this will allow the script running in the container to ask you for some prompts f.i.,
--rm to ask Docker to kill and remove the container as soon as the script has been executed (otherwise you'll have a lot of exited but not removed Docker containers; you can check this by not using the --rm flag then running docker container list on the console),
-v %CD%:/app to share your current folder with a folder called /app in the Docker container,
-w /app to tell Docker that the current directory, in the container, will be the /app folder
then signumtemporis/fpc:cross.x86_64-win64.slim which is the name of the Docker image to use and, finally,
Hello.pas i.e. our source file (the fpc image seems to not request to specify the fpc executable; just the source file).

Convert a number to a byte

Let's try something a little more complex than just Hello world; a conversion function to determine the byte representation of a positive number (please be comprehensive; it was a school assignment 😄).

I wrote this function in 1992 (published in the SWAG in 1997): https://github.com/cavo789/swag/blob/master/Byte2Bin/files/source.pas

Create the Byte2Bin.pas file on your disk with this content:

Function Byte2Bin (Chiffre : Byte) : String;

Var I, Temp : Byte;
    St      : String;

Begin

   St := '';

   For I := 7 Downto 0 do Begin
       Temp := (Chiffre and (1 shl I));
       If (Temp = 0) then St := St + '0' Else St := St + '1';
   End;

   Byte2Bin := St;

End;

begin
    WriteLn( Byte2Bin(197) );
end.

This program converts a single byte (a number from 0 to 255) into its binary representation as a string of 0s and 1s. It does this by checking each bit position from highest to lowest, and builds up the result string one character at a time.

SHL for Shift left one position

The shl instruction will shift the number from one byte left.

And compiled it:

Powershell

$ docker run -it --rm -v %CD%:/workspace signumtemporis/fpc:cross.x86_64-win64.slim Byte2Bin.pas

Free Pascal Compiler version 3.2.2 [2021/12/10] for x86_64
Copyright (c) 1993-2021 by Florian Klaempfl and others
Target OS: Win64 for x64
Compiling Byte2Bin.pas
Linking Byte2Bin.exe
20 lines compiled, 0.1 sec, 32656 bytes code, 1508 bytes data
▋

As you can see in the source code, the idea was to show the binary representation of 197 and, yes!, it's indeed 11000101. Still in 2023.

Powershell