When it comes to booting up a computer, there are two main methods that modern systems use: Unified Extensible Firmware Interface (UEFI) and Legacy BIOS mode / Compatibility Support Module (CSM). These two modes have significant differences in how they operate, and understanding these differences can be crucial when setting up a system or troubleshooting issues.
Legacy BIOS
BIOS, which stands for Basic Input/Output System, is a type of firmware used during the booting process of IBM PC compatible computers. The BIOS firmware is built into the PC, and it’s the first software to run when powered on. The name originates from the Basic Input/Output System used in the CP/M operating system in 1975.
The BIOS provides hardware initialization and the booting process at system startup. It offers runtime services for operating systems and programs and manages data flow between the computer’s operating system and attached devices such as the hard disk, video adapter, keyboard, mouse, and printer.
UEFI
UEFI, on the other hand, stands for Unified Extensible Firmware Interface. It is a specification that defines a software interface between an operating system and platform firmware. UEFI is meant to replace the Basic Input/Output System (BIOS) firmware interface, present in all IBM PC-compatible personal computers.
In comparison with BIOS, UEFI has firmware that provides more advanced features. For instance, while booting, it can support a graphical user interface (GUI) and network connectivity. UEFI, unlike BIOS, can boot from drives of 2.2 TB or larger—in fact, the theoretical limit is 9.4 zettabytes.
CSM
CSM, or Compatibility Support Module, is a component of the UEFI firmware that provides legacy BIOS compatibility by emulating a BIOS environment. This allows legacy operating systems and some option ROMs that do not support UEFI to still be used on a UEFI-based system.
UEFI vs Legacy / CSM
The main difference between UEFI and BIOS lies in the details of the systems they support and how they go about booting them. BIOS uses the Master Boot Record (MBR) to save information about the hard drive data, while UEFI uses the GUID partition table (GPT).
A significant advantage of UEFI is its security feature called Secure Boot. It prevents the computer from booting from unauthorized/unsigned applications, protecting the startup process against bootkit attacks.
UEFI also supports recent security standards required by Windows 11 and previous releases such as Windows 10. These standards include TPM (Trusted Platform Module), VBS (Virtualization Based Security), and others.
Conclusion
In conclusion, both UEFI and Legacy / CSM have their uses and are essential in the booting process. However, UEFI is the latest technology and provides more advanced features than BIOS, making it the preferred choice for newer systems. But for older systems and certain specific needs, Legacy BIOS is still relevant and necessary.
Understanding the differences between these two can help users make informed decisions about their systems and troubleshoot effectively when problems arise.