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Comment by jonhohle | Hacker News Reader

I work on a matching decomp project that has tooling to recompile C into binaries matching a 28 year old game.

In the final binaries created by compiled with gcc 2.6.3 and assembled with a custom assembler there appear to be unused, uninitialized data that is whatever was in RAM when whoever compiled the game created the release build.

Since the goal is a matching (reproducible) binary, we have tools to restore that random data at specific offsets. Fortunately our targets are fixed

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What even causes this to happen? ie. what dev tool would add random data from RAM to a binary? Is this likely a bug or is there some reason for it like needing to reach a specific file size somewhere?

By accidentally writing out uninitialized memory contents to the file with the game still working. It's even worse in DOS era where there is no memory protection so uninitialized memory can contain data used by other processes, so for example parts of source code can get leaked that way. There's a big list of those in https://tcrf.net/Category:Games_with_uncompiled_source_code

Yeah, this was originally all DOS and Windows 3.1 utilities for writing programs that would run on MIPS. The data is small enough that it isn’t relevant, just not reproducible through standard build tools because it was never meant to be bitwise reproducible.

Simply calling write() on a C struct can do that, if there is any padding in the struct. Then, of course, there are bugs.

please do write more about it.

We use a tool named dirt-patcher[0], which was written for the project. It lets you write arbitrary bytes at specified offsets[1].

As far as we know at this time, they’re just uninitialized bytes that would have been padding for alignment or other reasons anyway. Maybe if we move to an official build tool chain we’ll find they are deterministic, but for now, we believe they are garbage that happened to make it into the final binary.

0 - https://github.com/Xeeynamo/sotn-decomp/blob/master/tools/di...

1 - https://github.com/Xeeynamo/sotn-decomp/blob/master/config/d...

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