About a month ago, my dear friend porchy made this post on his blog. Now it is 'common knowledge' in the arcade society, that bootlegs of Taitos Bubble Bobble doesn't play like the original. The reason why, is that the original board has a custom chip (know as the PS4...the one featuring a picture of Bub from the game on the silk screen) that shares the SRAM with the main CPU (a Z80). The PS4 is a 6800-based CPU also containing some memory in the form of ROM. Every now and then, the PS4 changes some bits and pieces in the SRAM, so that certain events occur in the game. The things that most people notice as wrong is enemy movements and that the sequence of the EXTEND-letters is not randomized.
But recently MAMEDev succeeded in reading the program from a PS4 by using a technique known as decapping. In short, you cut off the top of the IC, look at the silicon plate under a strong microscope, and manually read every single bit. This way they made it possible to make a perfect emulation of the original game in MAME.
Now this is where joaoljr, a brazilian member on the arcadeotaku forum, got the idea that it might be possible to make all those old bootlegs play correctly! He took the 6800 assembler-code extracted from the PS4 via decap, rewrote it to Z80 assembler, and excecuted it in the spare interrupt cycles of the main CPU just the like bootleggers back in he days. He calls the project REDUX, and you can read all about it in this thread.
Phew, that was a long intro...but when I had read all this, I started looking for a cheap bootleg, and soon found an untested one from a french seller on evilBay. From the auction photo, I made sure, that this was indeed a REDUX-able type of bootleg ie doesn't have an 68705 MCU.
When the board arrived, I did the usual visual inspection. It was obvious that this board had been made in a rush; some components had not been pressed correctly into the holes, before the board went into the flowsoldering machine (this is the worst example)
Next, I started dumping all the ROMs on the board, but when I got to the one labeled "2", I found this
however the colours looked a bit strange. Either the pinout I had found was wrong, or this was a unique type, where blue and red crossed. I switched the two wires, and got this
Perfect colours and the sound was fine too. Quickly I hooked up the rest of the wires
Now for the REDUX-part. I downloaded the package with the prepatched ROMs (linked from the thread), and programmed them onto 3 27C256 EPROMs. Slammed them into the board (I have to make some nice custom stickers };-P),
Meesa liiiike dat!!! Haven't played it to the end yet, but it seems to play just flawlessly };-P REDUX FTW!!! A special thnx to joaoljr for the big effort.