(warning: this post is [not-so] oldskool)

The other constant push of demoscene, besides the possibility to fit as much as possible into executables of ridicolous dimensions (see the 4ks) is going beyond hardware limits, push known also as "hacking".

The "hacking" therm, more than indicating the fraudulent incursion in a bank terminal or in other informatic system (as nowadays press seems to think), means instead having some hardware and making with it something generally considered impossible on that platform or something for which that platform was not created.

The 4ks and all the production with kbyte size are certainly a form of "hacking".

But there are episodes in the scene where the results obtained hacking the hardware are much more impressive.

For example, IBM PC has an internal speaker apparently capable of reproducing only squeaks and gurgles (you'll probably remember the first, deafening, PC games).

Well, in 1990 the legendary Future Crew release the first intro where they start to do something serious. This intro, "Yo!", plays digital music from the PC speaker. How do they do that?

It's "easy": PC speaker is linked to a power source that emits only 5V and 0V voltages. The 5V voltage brings the PC speaker cone to the maximum extension, while the 0V one brings it to rest. But, what would happen if something stopped the cone actually before reaching full 5V position? Obviously we are talking of million-th of second intervals, but thanks to this technique, the limits of PC speaker are actually surpassed, and the "internal squeaker" becomes able to reproduce digital samples.

Back in 1992, the same group, the Future Crew, brings down another barrier. The most common videocard at that time was VGA (do you remember Tseng ET3000?), whose chipset was able to display up to 256 colours.

With a technique called "copper" (from the Amiga chip with the same name) or, more commonly, "raster effect", in the "Unreal" megademo, they manage to put thousands of colours simultaneously on the screen. Is this magic? Is this real?

That's how it works: in VGA videocards, the 256 colours are stored in a sort of table called "palette", and the card reads this table when displaying something on the screen. The Future Crew coders intercept the electronic beam when it's passing from one line of the screen to another, and they change the palette actually when the beam is switching lines (and not drawing anything).
Again, the demoscene coders "stopped" time (the videocard refreshes the display at least 60 times a second, and it shows at least 200 lines in a screen. So the code that switches the palette has to run in a 10.000th of second timeframe).

That technique is not a novelty, anyway: it was used already on Amiga itself, and even on C64, when it gave the possibility to show more than the 8 sprites the hardware designers had planned, or even to do the famous "border opening" effect.

Sometimes, unfortunately, the hacking ideas start to lessen, since technology evolution surpasses those barriers that were introduced for cost reasons or simply because time wasn't mature yet. At that point, the democoders do a step backwards and try to put some limitations themselves. What would happen, for example, if we tried to do a demo without hi-resolution graphic modes, not even 320x200?

The textmode demos are the answer. Textmode demos don't use, as said, high resolution modes (the ones where every single pixel is addressed separately), but just stick with text modes, where the atomic graphical element is, indeed, a character. Feel limited? Not for the democoders: using the special graphic chars of the charset and the 16 different EGA colours, they create effects that recall trustfully the ones used in high-res demos and, more than else, having to draw less "pixels", they can produce more complex effects.

Sometimes the difference between textmode and high-res effects is highly thin: in the "Textro" demo by OTM, there is a scene with wireframe solids. The OTM's coder had first to explain the algorithm (remap the charset at every frame) and then even release the source code to the effect, because noone was believing that effect was textmode!

The same trick (textmode) is used in the recent "8088 corruption" demo, just to do realtime video on a 4.77Mhz IBM PC (!).

Do you remember any other hardware tricks that made "impossible" things possible on a computer? Write them in the comments!

posted by friol at 9/10/2007 01:00:00 AM - under: copper, hacking, textmode - comments? here (11)

Post più vecchio Post più recente