For a long time, it has been recommended that support for the BIOS should be discontinued.Currently, most UEFI firmware-based computers are still equipped with a Compatibility Support Module (CSM) to allow programs to be installed on computers that do not support UEFI. According to one document, however, this mode should be eliminated from all devices in the “Clients & Data Center” area as of 2020, so that they only start in UEFI mode.
Of the different UEFI classes then only the classes 3 and 3+ (with Secure Boot) are supported. Old software such as the 16-bit DOS operating system can no longer be started. In addition, many rescue tools and network programs are no longer supported, unless they have been adapted to the UEFI mode. Since it is often software that was developed years ago, it is unlikely that the programs will be rewritten again.
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In contrast, many modern Linux distributions already support the new standards. By not using the CSM option, computer hardware manufacturers have the opportunity to save time and money on testing their systems. In addition, the UEFI BIOS code becomes smaller due to the omission. With the increased use of Secure Boot, systems are also much safer.