I was able this weekend to get graphics modes 4 and 5 done. I've got DMA transfers almost done except I'm using a for loop which is probably too slow, but I can optimize later. The THUMB decoder and disassembler are done but there aren't any THUMB functions implemented yet. For those of you not familiar with the ARM it has two modes: ARM and THUMB. In ARM opcodes are 32-bit and can take many operands, in THUMB opcodes are 16-bit.
Hopefully I won't have that much work this week so I can get sprites done and make a small game(probably just tic-tac-toe or something).
For those interested in testing out mode 4 I made a little plasma demo. The palette isn't smooth so it doesn't really look right but it still shows that mode 4 works. Just download FatMan.zip and drag plasma.gba into FatMan.