Preface:

Yeah, I know it's not really the policy of DMGICE to cover grey area stuff like emulation, and we even have disclaimers about it, but there's just some stuff that's too cool to ignore.  Stuff like emulation of old consoles on the GBA- that just gets me stoked :)

So far, two emulation projects have surfaced, and more are expected to in the upcoming days and months.

The first is a port of an older PC-based NES emulator (LoopyNES), by noneother than Loopy himself.

The Second is an all-original Atari VCS (2600) emulator, by Otaku
 
 

PocketNES

I know what you're thinking right away- you've seen that NES emulator for the GBC, and you think that any GBA NES emulator will probably be just as slow, if not generally unplayable.  Well, wrong you are sparky!  Gone are the days of 3-frames-per-minute, we now have a full-speed NES emulator for the Gameboy Advance.  Yes, full speed.  60 frames per second with clock cycles to spare.  Believe me, I've tried it.

Okay, before I get drool all over the keyboard, there's some things that you should know.  First of all, the emulator is written entirely in assembly language code, for the most efficient use of all clock cycles available to the CPU.  Also, the NES screen had a resolution of 256x256, meaning that the picture doesn't really fit on the screen of the GBA, with it's 240x160 resolution.  Right now, all you see is the upper-left portion of the screen- which in many cases doesn't even really effect gameplay too badly in some games (Megaman 2 runs fine- but that's probably because I remember where everything is :P).  Anyways, a further release is expected to skip scanlines and center the screen, or do something to get the game fully visible on the screen.  With several first and second generation games, gameplay was flawless- I already mentioned Megaman 2.  Megaman 1 and 2, Metroid, and a host of other titles worked without glitches at all, several worked with glitches (mainly games with scrolling and windows), while most still don't play at all.

Anyways, here's a list of the features directly from the website:

Multiboot
Mapper 1 games (with no CHR)
Mapper 2
Scrolling (sorta)
Minor 6502 bugfixes
Moved nes rom to the end of the file

Alright, so there you have it.  This is going to be one project to watch!  Visit the official PocketNES website for pictures and information on how to get it running on a real GBA.

PocketNES website
 
 

GBAVCS

I don't know about you, but the Atari 2600 was my bread-and-butter for several years (along with my vectrex :P) from the age of 3 until I got my NES.  I have many fond memories of playing various versions of arcade classics, as well as the obligatory Yars Revenge marathons.  Those were the good old days!  Anywho, soon enough we may be able to relive them in the palms of our hands thanks to Otaku and his mad programming skills.

Like Loopy, Otaku has written his emulator entirely in Assembly language (a feat that still scares me) to achieve a respectable ~40 fps, with room for the emulator to reach the ultimate 60fps boundary.  Like PocketNES, there is no sound, and only digital joystick support is available, but this is a terrific start to the project.  However, this emulator currently only runs on emulators such as VGBA for Windows which, as the auther put it, is probably due to something obvious.

Here's the list of features thus far:

Rom bankswapping for 2,4, and 8Kb games
3 different screen modes (vertical and horizontal orientation)