See the top level README for information on where to find the schematic and programmers reference manual for the ARM processor on the raspberry pi. Based on some web searches the gpio is based at address 0x20200000 not 0x7E200000 as shown in the documentation. I have to go figure that out. Apparently what they have done is have the thing boot using the gpu in the part, dont bother, leave it be, they are not going to let us program it, not for a while at least. There is I assume a gpu bootloader programmed in the chip, an OTP ROM based on something I read. This searches for the sd card, and I assume loads another gpu bootloader from that, at some point, the arm memory is loaded from kernel.img on the sd card, then the ARM is released from reset. This simple example, sets up a small stack, not much to this program, and I have not read up on how much memory is actually in this space. It then enables gpio16 as an output. Then goes into a loop that sets the gpio, waits, resets it, waits, repeat. gpio16 is shown on the schematic to connect to the OK led. One of the bank of leds on the corner near the audio out and host usb ports. The blink rate for me is a few blinks a second perhaps. Future examples will get use a timer if there is one and narrow in on the clock setup, etc. Copy the kernel.img file to the sd card, overwriting whatever you had there. You may wish to backup the one you had there before you started playing at this level. The beauty of this boot design is that it is not (easily) brickable, so far as we know. With ignorance being bliss I will say it is not brickable. Second we have no flash on the card we always run out of ram as far as we are concerned. Making the board cheaper among other things. The negative side to this is the development process is very painful. It goes back to the GBA or NDS days, actually it was more painful then. Here you unplug the rpi, pull the sd card out, put the sd card in an sd card reader, plug the sd card reader into the computer, wait or mount, copy kernel.img to the sd card, wait or sync, unmount the sd card, remove the sd card reader, remove the sd card place in rpi, plug in rpi. And repeat with each new build. I hope to get the uart figured out and a bootloader so that I/we only have to do that once, and from there on power off, power on, load the program over serial. Not to dissimilar to the arduino experience.