Does someone know exactly how work MMC4's graphic latches ?
It's very badly explained in Firebug's doccument and I want to know if someone has more knowledge about this.

Bregalad, Ent from the woods of Fangorn

The MMC2 and MMC4 function almost EXACTLY the same as a whole. The only known differences (so far) are as follows:

MMC2 - 40-pin shrink-DIP; 8KB swappable bank at $8000-$9FFF, last 24KB hardwired at $A000-$FFFF
MMC4 - 44-pin TQFP; 16KB swappable bank at $8000-$BFFF, last 16KB hardwired at $C000-$FFFF; support for 8KB SRAM at $6000-$7FFF, always enabled.

Both chips have the same CHR bankswitching setup (with latches in BOTH banks).

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Quietust
P.S. If you don't get this note, let me know and I'll write you another.

This may help:

http://oldnes.sourceforge.net/doc/mmc2.txt

it's MMC2, but my understanding is that MMC4 is identical (except for the differences Q mentioned in his reply).

In other words, when particulars tiles $fd or $fe are drawed on the BG, another lached CHR-Bank will be automatically switched ! That's really funny. I understand now why MMC4 games are able to show 2 Chr Banks at once on the screen. This is pretty easyer than do mid-scanline bankswitching.

Bregalad, Ent from the woods of Fangorn

Almost; it isn't specifically when they are fetched for background - it's when the tile is read by the PPU at all, so it can be used by background AND sprites.

The main advantage for this is displaying text boxes on the screen without having to keep the character set swapped in the entire time, allowing for backgrounds with much more detail.

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Quietust
P.S. If you don't get this note, let me know and I'll write you another.

Well I just implimented MMC2 as the info from this post implies:

http://nesdev.parodius.com/cgi-bin/wwwthreads/showpost.pl?Board=NESemdev&Number=1158&page=&view=&mode=threaded&sb=#Post1158

Mike Tyson's Punchout works terrifically.