The making of Don Matrelli’s Legacy. Part IV: putting it all together

In the past episodes we have seen how Grand Prix Circuit (GPC) was partly reverse-engineered, and how to build new tracks and cars. All the main components of my GPC mod have been described: for the final phases of the building process I’ll return to a more diaristic form. The launch vehicle Already from the start of the project I needed a practical method to inject the new data into the game: patching thememory via DOSBox debugger is only good for small experiments. I had not yet figured out how to decompress the executable, so I could not edit it. But I had an alternative plan: instead of modifying the GPEGA executable, my mod would live as separate TSR program to be run before the game. The TSR would install a callback in memory, and ask DOS to execute it whenever interrupt 10h (“Video BIOS Services”) was called. GPC calls this interrupt twice at game start: on the second invocation, after GPC had written its data in memory, the callback would replace them with the modded ones. I…