I've been debugging callbacks and now they work on AMD64. I also cleaned up the x86 assembler and extended it to support the full range of x86 addressing modes. On the Mac side, Objective C exceptions are now caught by the Factor runtime and reported to Factor code in the same manner as Factor exceptions; this makes debugging Cocoa code much more pleasant.
I would like to port Factor to MacIntel, but I don't have such a box; if any reader of this weblog does and would like to offer me an ssh account, please let me know! Perhaps it won't even take any work; the x86 compiler backend exists, and Factor does run on PowerPC Macs. The only issues I can forsee are in the FFI; if the OS X ABI is sufficiently different from Linux/x86, changes will need to be made.