Some intrepid soul has ported the SIMH emulator to the GBA and is using that to run the PDP-6 version of UNIX v6. Actually, looking at the stats of the (ARM-based) GBA, it would have been eye-poppingly advanced back in the late 60s. There's a
great article about it at Kernelthread.
Update: Thanks to Vanders on setting me straight on which version of UNIX was actually ported.