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. There is a free-running 64 bit timer, super easy to use, just read it. Based on a couple of experiments, without messing with anything it appears that the timer is runing at about a megahertz, 1 million ticks per second. I am guessing it is divided down from the 700MHz somewhere, will dig deeper. Hmm, there are comments in the manual about the system clock coming up at 250MHz. I wonder if this system timer is 250MHz/256 = 0.976MHz.