Some programs work, including green-card and some HOpenGL programs. GHC, compiled with my registerized version, crashes. At some point, there is pointer on the Stg Stack that points into data space. However, it doesn't point to a closure. In points to a place just after the last data symbol in of one GHC module, and before the next (there's quite some stuff between those two modules, and I have no idea what the linker puts there). The word at that position is definitely not a valid info pointer, it points about 300 megabytes above the heap (into unmapped space). MacOS X doesn't yet support watchpoints, so I still have no idea "who" put this strange pointer onto the stack. Under what conditions are pointers into data space put on the stack? Any ideas how I could go about debugging this? How can I possibly ever find out what code is putting this nonsense value on the stack? For now, I'll try to find some smaller programs that crash, too. Wolfgang