I have a MacBook (which is a Core 2 Duo) running Mac OS X 10.4. On this platform, gcc makes 32-bit executables by default, but if you give it - m64 it makes them 64-bit. I've tried it, and it works. gcc gives its version as "i686-apple-darwin8-gcc-4.0.1 (GCC) 4.0.1 (Apple Computer, Inc. build 5367)" But 64-bit doesn't work with my ghc installation (6.8.2 from the binary distribution on the downloads page). If I compile a "Hello World" program with -fvia-c -optc-m64, it fails, saying -- Warning: retaining unknown function `_siC_entry.eh' in output from C compiler Warning: retaining unknown function `_Main_main_entry.eh' in output from C compiler Warning: retaining unknown function `_ZCMain_main_entry.eh' in output from C compiler Prologue junk?: _siC_entry: LFB142: subq $8, %rsp LCFI0: -- I don't need to solve this, but I thought I should make sure it's known. Jay