Atari 7800:Region Information
Although NTSC and PAL Atari 7800 games look identical and despite certain games being easier to find in particular regions, there are a couple of complicating factors to consider.
NTSC 7800 consoles feature an security measure in their BIOS in order to (presumably) deter unauthorized third party games, a problem which plagued its predecessor, the 2600. If the correct 960-bit encrypted "signature" is located in the game code, the system will allow the game to run; if not, the system will go into 2600 backwards compatibility mode and the game will not play. PAL 7800s do not have this encryption scheme and PAL 7800 games also do not have this section of code. Because of this, PAL 7800 games will not run at all on a stock NTSC 7800. This can be bypassed by swapping the BIOS with one that does not perform this check, such as the PAL BIOS or the one used in development units.
However, the BIOS is part of the problem on NTSC 7800s as PAL games were coded specifically for PAL hardware. Many PAL games will either exhibit major graphical glitching or simply not run on NTSC hardware. On the other hand, PAL 7800s have an easier time running NTSC games, which is rather fortunate as a number of 7800 games did not receive a PAL release. A sizable number of NTSC games will either run perfectly or only have minor graphical issues on PAL consoles, and only a few will not run.
Due to the number of parts needed and complexity involved, there is no modification which will allow either region console to perfectly play each other's games. Therefore, it is much more cost-effective and easier to simply buy an NTSC 7800 for NTSC games, and a PAL 7800 for PAL games.