updated 05:25 pm EDT, Tue September 24, 2013
Update fixes Boot Camp-related Windows install issues
Hours after the announcement of the Haswell-powered iMacs, Apple has released an Extensible Firmware Interface (EFI) update for the new models. The update brings the firmware on the new models to version 2.1, and "addresses an issue which may cause a black screen to appear when installing Windows 7 or Windows 8 using Boot Camp Assistant if both an external optical drive and USB thumb drive are connected to the system." Additionally, the update guarantees that the system boots by default into OS X after a Windows 8 install.
The EFI specification defines a boot manager, a firmware negotiator that is in charge of implementing the OS loader and all necessary drivers. The boot configuration is controlled by a set of global NVRAM variables, including boot variables that indicate the paths to the loaders. Apple's implementation of EFI gives the user a choice of OSes to boot into upon system startup, if desired.