Autopilot

It's true that sometimes when you are blocked on a problem you just have to leave it aside for a while to look at it from a new perspective.

Yesterday I spent much of the night fixing the tile decryption for the RISE10 chip, which is used by Raiden Fighters 2. With the experience gained on the earlier version of the chip this was much easier, and I could fix most of the problems reasonably quickly. However, there was one bit in one layer that just didn't want to work.

The algorithm, now that it's understood, is really clean and simple, following intuitive patterns, but that bit didn't fit - to partially decrypt it, I had to use the wrong bits from the tile code, and pick a carry from the wrong bit of the encrypted data. It just didn't make sense.

I decided to leave it alone for the night, but literally a few seconds after turning off the monitor and going to bed, the obvious answer came to me: that bit had to belong to a different plane! I must have been fetching the encrypted data from the wrong place. And indeed, this morning I checked, and bit 16 of the source data was used twice, while bit 15 was not used at all. Fixing that was all that was needed to completely fix the tile graphics.



There aren't many screenshots that I can take because the driver also has problems selecting the tile bank (this is an emulation problem not related to the encryption). Plus of course, sprites are still not decrypted correctly.

I still have to decide what to do next - I could either look at the RISE11 encryption, used by Raiden Fighters Jet, which will be a bit more work than the others because the current driver only decrypts two of the six planes (for the other games, the decryption was already very close). However, now that I have a good understanding of the algorithm it shouldn't take long to do it, even starting from scratch.

The other thing I could do is fix the tile banking problems in Raiden Fighters 2. I haven't looked at the driver yet, it might be something simple.