When a PC boots with an ACPI BIOS, what does it exactly do?

I understand that the point of ACPI is to allow the OS to control hardware resources and power management but before the OS is loaded does ACPI configure just the devices needed to boot and then let the OS configure the rest?

If the OS wants to re-asign hardware resources does it store this information in the ACPI tables so that the next time the system is booted it assigns them how the OS wants?

The ACPI driver asks the PCI bus driver (Pci.sys) to enumerate devices on it's bus once the OS is loaded, how are these devices configured whilst the PC is booting when it doesn't have other bus drivers?

Any help with any of the above questions would be greatly appreciated.

Thanks.