[Sebastian Mihai] is a prolific programmer and hacker with a particular focus on retrocomputing and period games, and this latest hack, adding new gameplay elements to Capcom’s Street Fighter II – Champion Edition, is another great one. [Sebastian] was careful to resist changing the game physics, as that’s part of what makes this game ‘feel’ the way it does, but added some fun extra elements, such as the ability to catch birds, lob barrels at the other player, and dodge fire. The title screen was updated for each of the different versions, so there is no doubt about which was being played. This work was based on their previous hacks to Knights of the Round. Since both games shared the same Capcom CPS-1 hardware, the existing 68000 toolchain could be reused, reducing the overhead for this new series of hacks.
Binary modification program flow
Obviously, without access to the game’s source code, the hacks (all seven of them!) were made by binary modification, first learning about how the original program stores aspects of the game and hacking in a little hand-grafted assembly here and there to sneak in the extra elements without interfering too much with the original code operation. [Sebastian] stripped out some title screen effects to speed boot time and removed some in-game graphics, such as the score and the ‘insert coin’ images, to free up some graphical tiles to reuse for the new elements. [Sebastian] first needed to understand the game code, which meant disassembly and hand annotation of the entire binary, which was done using the MAME debugger. As the linked article demonstrates, sa ..
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