GBA compression is handled one of two ways.

1. Hardware decompression, using the GBA's own built-in LZ decompressor.

2. Software decompression, using an ad-hoc proprietary method.

Option 1 is naturally the easy way, since the GBA's compression methods are probably all well-documented. The actual decompression part might take a little effort to find, though: it loads certain registers with the necessary values and then makes an SWI call.

Option 2 is a little harder, since you'll need some basic programming knowledge to figure out how the compression does what it does.

Whichever method you choose, you will have to deal with the compression somehow to get at the original script. Writing a decompressor is easy once you have the ASM equivalent in your hand. Writing a recompressor, however, is hard. I mean, anyone can make a sausage machine, but it's some trick to make one where you can turn the sausage back into the pig afterwards. ^_^;

Your best weapon for GBA hacking, IMHO, is VBA-SDL-H. It's a GBA emu with crappy sound and a killer debugger. Problem is, the debugger is command-line based, so you might have problems if you're uncomfortable working with a DOS prompt or the like. You can use VBA-SDL-H to watch when the game loads compressed data from the ROM, and trace it to see what the game does to the data afterwards. That's how you hunt down your decompression routine. (Of course, you'll need to know where the compressed data is stored in the ROM, which is its own special sidequest...^_^;)

Ok, Now i strat to debug ffv . Now i use Tracer-vgba to trace command in gba and i got some trace.log. Could some body tell me what is the command of arm7 that perform which button that user press? and how do i know that's 'A' button.

i've read read and read. But still struck . I'll keep trying.
Now, i've relized that knowlage from nds hacking doen't help me to hack gba anymore . 

after i read documents, i found that gba has bios function to decompress data for LZ77, the command is "swi $11" . so i use program "Visualboy Advance Tracer" to keep log of asm command. but when i search in log file (it's about 100,00 lines) i just found only one line of "swi $11". Did i do something wrong?

another question, Can i dump vram or wram and then search for dialog text such as "Hell" in dump file.

Thanks

Really ? so , can i use hex editor to search text or dialogues in this file?

How can i seach dialogues when it was compressed?

You can download the latest version any time for one year. You don't have to buy further upgrades for every new version which is very nice. The GBA part is pretty much bug free anyway, recent upgrades were all for DS emulation.