Depending on the CPU / APU model, the highest boosted frequency PState usually has a C6-state requirement.
For example on FX-8370 which has 4.0GHz (P0, base), 4.1GHz (Pb1, boost) and 4.3GHz (Pb0, boost) states, the C6-state requirement is set to four. This means that the highest PState (4.3GHz, Pb0) will not activate unless half of the cores are currently in C6 active state.
If you disable C6-state this condition can obviously never be met, and the highest boosted state will never activate.
In this case the CPU will operate at the highest frequency PState which doesn´t have the C6-state requirement (4.1GHz Pb1).
Likewise if you disable APM(Master) feature from the bios, it will essentially disable Turbo.
Killing APM will kill the power management and after that the CPU no longer knows how much power it consumes.
Requirements for Turbo (CPB):
- CPB Enabled
- C6 Enabled
- APM Enabled
Failing to meet any of those will render Turbo either partially or complete useless.