You need to go to your manufacturer and get the manual. The chip may or may not dictate the complete API. Generally, there is some memory location which, if you write it it, turns on the pin. But as you note, if there are two potential voltages, that might be fixed by the board maker, or software controlled.
To give you an example, on an ARM-9 board (TS-7200) you have to first write to one location, which sets up the page you are writing to, then write to the location. It's 3 or 5 lines in C, and it's not dictated solely by the CPU. You'd better have the model number of the board too, because even for the manufacturer, it could be different per board.