I have Asus X200MA Laptop with UEFI Firmware & Windows 8.1 pre-installed in UEFI mode (CSM Disabled). Now if I turn CSM on, I find that it still successfully boots into Windows 8.1 which otherwise uses GPT partition scheme.
I am curious to know how is this possible? if I correctly understand, CSM emulates BIOS Mode booting in the firmware. So it should look for MBR disk and since it does not find any should not allow booting Windows 8.1 off GPT.
Thanks.
Answer
In most firmwares, enabling CSM mode doesn't fully revert the system to 1990's behavior – it merely adds special "BIOS disk" entries to the regular boot menu. If those entries fail, however – e.g. if the disk has no bootloader 'signature' – the firmware still keeps trying the next entry, until it eventually reaches the "UEFI: Windows" item.
Remember how even actual BIOS systems have a "boot selection menu" for choosing which device to boot from – they're not forced to stick to a single boot source. So UEFI CSM doing the same is nothing new.
(Also, BIOS does not typically care about actual partitions, just the bootcode, and a GPT-partitioned disk can very well contain BIOS bootcode in the 'protective' MBR. And vice versa, EFI bootloaders can live on MBR-partitioned disks.)
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