In Windows 8 long gone is the ability to interrupt the boot process via Shift+F8 or F8 and getting into the BIOS configuration (in UEFI enabled systems), because the boot process now is too fast that is almost impossible to detect a keystroke.
According to Chris Clark, program manager at Microsoft’s User Experience team, says that there is only a 200 ms window to be able to interrupt the startup sequence before the Windows boot manager detects a different pathway to start the OS.
The software maker explains the solution to access the new boot options menu in the forthcoming version of its popular OS:
1. All the advanced boot options, even to access the BIOS (in new capable devices) has been grouped together in a single menu.
2. The PC will automatically go into the boot options if Windows 8 won’t start correctly.
3. Third and last, Microsoft is making sure that users can access the options even though there is nothing wrong with the operating system from almost any location.
Basically, you will lose the ability to change the boot pathway to start in safe mode or recovery environment and BIOS settings, when the PC goes through the POST and Windows boot process kicks-in, but you’ll still be able to get to them from the General > Advanced setup option within Windows 8 or by holding down Shift and clicking Restart. If for any reason you computer does not successfully complete the boot sequence, Windows will be smart enough to notice and load the boot settings automatically.
Now in legacy hardware with traditional BIOS, Windows 8 will not provide access to make changes to booting into the PC’s firmware or change the boot order and start directly from another device (e.g., USB drive, network, etc.). The user will access those settings in the traditional way via F2, Esc, or another keyboard key assigned by the hardware manufacture.
The company says that these new changes will help users to have a consistent access to troubleshooting tools, methods for accessing the firmware BIOS configuration, and a straightforward method for booting to alternate devices like using a USB drive, while allowing a fast boot into the operating system.
Source Building Windows 8 blog