{"id":22,"date":"2014-03-19T16:20:35","date_gmt":"2014-03-19T16:20:35","guid":{"rendered":"https:\/\/blogs.scummvm.org\/josejx\/?p=22"},"modified":"2022-05-21T16:23:56","modified_gmt":"2022-05-21T16:23:56","slug":"figuring-out-what-to-do","status":"publish","type":"post","link":"https:\/\/blogs.scummvm.org\/josejx\/2014\/03\/19\/figuring-out-what-to-do\/","title":{"rendered":"Figuring Out What to Do"},"content":{"rendered":"

Continued from\u00a0the previous entry<\/a><\/i><\/p>\n

In this post, I’ll show you what my work flow generally looks like for reverse engineering. In the past, I’ve worked on a few reverse engineering projects like the Broadcom BCM43xx<\/a> and the Collie SD Card<\/a> interface for Zaurus. In any reverse engineering project, the first thing to figure out is where to begin! For this post, I’ll take a stubbed function from the ResidualVM code, explain how to find the original implementation, figure out what it does and then re-implement it.<\/p>\n

After getting EMI running, I noticed that there were a lot of debug messages about stubbed functions in the console window.<\/p>\n

\"\"<\/a>
Console window, filled with debug messages<\/figcaption><\/figure>\n

Picking one at random, I decided to look into the function Lua_V2::SetActorLocalAlpha<\/i>, but first, I needed to do a little bit of research!<\/p>\n

Understanding what the structure of the program is before diving into the assembly is usually a good idea. From the documentation at the ResidualVM wiki<\/a>, I saw that Lua<\/a> was the scripting language running the game and that EMI’s engine was structurally similar to the one used in Grim Fandango. Let’s take a quick look at the source code for ResidualVM and investigate the structure some more.<\/p>\n

In engines\/grim\/emi\/lua_v2.h<\/a><\/i> (line 39)<\/i>, we can see the list of Lua script functions that the engine provides. The actual code that implements these functions can be found in the rest of the files in engines\/grim\/emi\/<\/i>. Of note, the function we’re interested in, SetActorLocalAlpha <\/i>can be found in engines\/grim\/emi\/lua_v2_actor.cpp<\/a><\/i> at line 38. Helpfully, this code is partially completed, waiting to be finished!<\/p>\n

So, to summarize what we have so far:<\/p>\n