August 16th, 2013

Second Reality Code Review: Part 1 (Introduction)

I love you Hoi-En.
On July 23, 2013 the source code of Second Reality was released. Like many, I was eager to look at the internals of a demo that inspired me so much over the last 20 years.

I was expecting a monolithic mess of assembly but instead I found a surprisingly elaborated architecture, mixing several languages in an elegant way. The code is something like I had never seen before that perfectly represents two essential aspects of demomaking :
As usual I have cleaned up my notes into an article: I hope it will save some a few hours and maybe inspire others to read more source code and become better engineers.

Part 1: Introduction
Part 2: Engine
Part 3: Demo Interrupt Server
Part 4: Dev Vs Prod
Part 5: Parts

The Demo

Before starting with the code, here is a link to a HD video capture (by Michael Huth) of the legendary demo. Nowadays, it is the only way to fully experiment the marvel without graphic glitches (even DOSBox cannot run it properly).

First contact with the code

The source code is hosted on GitHub. It is one git command away :

   git clone


At first the content is confusing: 32 folders and a mysterious U2.EXE that won't run under DosBox.

The working title of the demo was "Unreal 2" (the first "Unreal" was Future Crew's previous demo, released for the first Assembly in 1992). Only later during development the name was changed to "Second Reality". This explains "U2.EXE" but not why it doesn't work...

Runnig CLOC provides interesting metrics:

    Language                     files          blank        comment           code
    Assembly                        99           3029           1947          33350
    C++                            121           1977            915          24551
    C/C++ Header                     8             86            240            654
    make                            17            159             25            294
    DOS Batch                       71              3              1            253
    SUM:                           316           5254           3128          59102


Running the demo

It is hard to figure it out but it is possible to run what was released via DosBox: You have to rename U2.EXE and run it form the right place. Once I learned about the internals of the code, it made a lot of sense:

        CD MAIN
        CD DATA


Et voila !


Back in the 90s, the demo was mostly distributed via floppy disks. After unzipping, two big files would be installed : SECOND.EXE and REALITY.FC:

    .               <DIR>         01-08-2013 16:40
    ..              <DIR>         01-08-2013 16:40
    FCINFO10 TXT           48,462 04-10-1993 11:48
    FILE_ID  DIZ              378 04-10-1993 11:30
    README   1ST            4,222 04-10-1993 12:59
    REALITY  FC           992,188 07-10-1993 12:59 
    SECOND   EXE        1,451,093 07-10-1993 13:35
        5 Files(s)     2.496,343 Bytes.
        2 Dir(s)     262,111,744 Bytes free.

Coming from a game developer background I always expected the big picture to be :

But after reading MAIN/PACK.C I discovered that I was completely wrong : The "Second Reality" engine is just a Loader and an Interrupt server (called DIS). Each scene (also called "PART") during the demo is a full self-contained DOS executable. Each parts are loaded by the Loader and started one after an other. The PARTs are stored in encrypted form at the end of SECOND.EXE :

This architecture offers many advantages :

Recommended readings

The three pillars to understand Second Reality source code are VGA, Assembly and PC architecture (PIC&PIT Programming). Here are incredibly helpful links :


Second Reality Engine



Fabien Sanglard @2013