ROMlib:

ROMlib is ARDI's implementation of approximately 1,200 Classic Macintosh OS routines. The choice of routines to be included in ROMlib was based both on the order in which routines were added to Mac OS by Apple and examination of popular programs to see which routines they depended on. ROMlib is the heart of both Executor and Carbonless Copies.

ROMlib routines have to reproduce the exact behavior of their corresponding Mac OS routines. Macintosh applications rely on the layout of data structures whether the data structure is documented or not. Macintosh applications also "patch out" routines by replacing a Mac OS (or ROMlib) routine with their own implementation. Such patching imposes even more compatibility restraints.