Well, fuck:

http://board.byuu.org/viewtopic.php?p=57219#p57219

> Why would $3804 return D7 set?

d7 is the "ARM ready" flag. It is set in registers.hpp:Bridge::unsigned operator(). The flag is actually set to true inside ARM::enter(), after the reset phase completes.

If you set $3804.d0 to 1, it disables the ARM CPU. When you set it back to 0, the chip resets itself and then after it's done, will set the d7 flag to ready again. The chip is enabled at power-on, it does not need to be turned on manually.