Well.... secondly, the bc controls boost, but first and foremost the wastegate is your boost controller. The wastegate is what essentially controls boost by the tension of the spring inside of it (external WG). If you don't have a boost controller at all then the boost is totally controlled by the WG. This works by the amount of pressure that builds up from the exhaust ports on your head into the turbo. Once the pressure reaches a certain PSI (or BAR), the WG spring can no longer keep the WG closed so it opens, allowing the extra pressure to bleed off either to a section of exhaust piping after the turbo or to open atmosphere.

When using a manual or electronic boost controller, you can boost up to approximately four times the installed WG spring PSI rating - I was able to hit 16 psi on my 4 psi spring and upgraded to a 10 psi spring. These extra boost controllers work off of vacuum/boost pressure and hold the wastegate closed so that more pressure is able to build, thus creating more boost pressure.

It's a lot to digest, but I hope I've enlightened you a bit.