These examples are for Raspberry pi 2 (I don't have a pi or pi zero). It would be trivial to recompile these examples for pi, just change the first byte in GPIO registers from 0x3F to 0x20 and the target architecture from armv7 to arm.
Major credit to David Welch [email protected] for providing bare-metal examples in C at http://github.com/dwelch67/raspberrypi