Script resources – a dyad in the Force, as it were

ZvikaZ recently ran into an issue trying to hack Quest for Glory 1 VGA where they edited a particular script, and it worked fine, but when they then exported the .scr file and put it in a clean QFG1 folder, it broke in a particular way. One particular phrase stood out to me in particular:

There are ‘ch’ strings instead of the numerical values

I had a feeling what the problem might’ve been when I started reading the post but when I saw that part I knew exactly what happened.

Quest for Glory 1 VGA is an SCI11 game. That means the scripts are split up into .scr and .hep pairs, and ZvikaZ only copied the one file instead of both. One of them contains the actual script bytecode, but the other contains the amount of local variables, their default values, information on all the objects in the script, and all the text string literals in the script. It’s called a heap resource because that’s where it’s loaded.

Originally, the script and heap resources were one and the same. When a given script needed to be loaded, it would be loaded into heap memory and kept there until unloaded. And as explained before, a saved game is basically a compressed dump of the entire heap memory area, while hunk space contains all the other resources that the scripts, in turn, refer to. Now imagine for a second a script resource with a single class in it, with a single particularly big method, so that a mere fraction of the script resource describes the class, and contains any near strings and such, and all the rest of it is bytecode. Once loaded, the bytecode can’t be changed — only the class properties and any local variables can be, but all of that bytecode is still part of the heap. There’s only so much heap space available to a game, so as long as that script is resident, that bytecode will take up precious space.

SCI11 split the script resources up so that the bytecode parts would be kept in hunk space instead, swapped in from disk when actually needed by something from the script definitions in heap space. All that space taken up by PMachine bytecode is suddenly no longer part of the heap and this bad boy can fit so many script resources at once. And if your scripts use far text instead of near — text resources referenced by a module/line tuple that get loaded into hunk space, instead of "quoted strings like this" that are part of the script’s heap resource) anything those scripts try to say automatically also doesn’t take as much space. You trade a two-byte pointer for a four-byte tuple, but those numbers in turn may refer to a string of who knows what length. Savings!

ZvikaZ’s target was the script resource for QFG1‘s character creation screen, whose first class is a Room named chAlloc. That name appears in the heap resource. When ZvikaZ changed the script code and recompiled, the heap resource had its contents changed, including where exactly in the file the room’s definition started. Whatever mixed-up monstrosity resulted when ZvikaZ then tried to run the altered 203.scr against an untouched 203.hep didn’t function and notably printed ch instead of numerical statistics.

I’m honestly a little impressed it didn’t “oops” on the spot.