XPConnnect

XPConnect is the mechanism whereby XPCOM classes written in C++ can be accessed from javascript, and vice-versa. See XPConnect pages.

On the platform-specific front, there needs to be code to call the nth virtual method of a given pointer-to-object with an arbitrary-ish list of parameters. This requires assembler.

For emx, the code is written and will appear in /mozilla/js/src/xpconnect/emx_invoke.s (or something like that).

VACPP uses the _Optlink calling convention with C++ methods, and it doesn't seem you can persuade it not to. For XPConnect to work in VACPP builds, there needs to be some _Optlink-friendly invocation code.