Some of the arm instruction set manuals seem harder to come by than others. Recently I reverse engineered one of the firmwares of the ImmersionRC Vortex 150 racing quadcopter (it uses an stm32f3 chip, arm cortex instruction set). It was pretty hard to come by a copy of the full instruction set manual - i just assumed that kind of thing would be a quick google away. Eventually i got there and made my changes but it was harder to get going than i expected but for different reasons than i was anticipating.