When it comes to booting options, UEFI is the preferred firmware for modern computers with large drives.
UEFI comes with an integrated feature called CSM mode which offers legacy BIOS compatibility for older operating systems. However, there's a catch as to whether you should use it or not.
Recently, my cousin was having trouble booting into Windows on his PC. When I checked, he had a non-UEFI capable operating system. All we had to do was enable CSM and his computer started booting like normal.
Don't worry if it's getting too confusing right now!
I'll explain everything in this article, including the difference between CSM Boot Mode vs. UEFI Boot Mode, and the exact scenarios you should be using them.
Check out this side-by-side comparison table to know the key differences between CSM and UEFI BIOS modes.
|Maximum Disk Capacity||2.2 terabytes (TB)||9.4 zettabytes (ZB)|
|Secure Boot Support||No||Yes|
|Disk Partition Type||Master Boot Record (MBR)||GUID Partition Table (GPT)|
Before we dive into UEFI and CSM boot mode, let's briefly discuss the fundamentals of BIOS.
The traditional BIOS (Basic Input/Output System) can be seen as the "firmware" of your PC. It's the first piece of code that runs on the computer when it's powered on.
What it does is execute the OS boot loader, which then starts all the operating system's programs and processes. The BIOS initializes all the hardware components in your PC, such as microchips and peripherals.
Now, UEFI (Unified Extensible Firmware Interface) is nothing but a modern replacement for the legacy BIOS system. It began to replace BIOS around 2007 due to its severe limitations.
For example, legacy BIOS works on a system called MBR (Master Boot Record) which can only handle partitions up to 2TB. Nowadays, most modern computers use UEFI boot mode rather than legacy BIOS mode.
Along with this transition, a new component called "CSM (Compatibility Support Module)" was introduced.
CSM stands for Compatibility Support Module. It's a utility included with the UEFI firmware that offers backward compatibility for modern computers.
Using CSM boot, you can perform legacy BIOS style booting on a modern UEFI-based system. This can come in handy if you're using an older operating system or want to boot from an old storage drive that doesn't support UEFI BIOS mode.
Remember, BIOS and UEFI are two types of computer firmware. However, CSM is just a feature provided in the UEFI setup menu.
If you're using a new computer with a recent version of the Windows operating system, it likely has CSM disabled by default.
Now let's look at the significant differences between CSM and UEFI, and understand which one is the right choice for you.
CSM is compatible with legacy hardware and software since it's based on the old-school BIOS. If you have a PC with very old components, you probably need to use legacy BIOS booting.
On the other hand, UEFI is a modern standard designed to take advantage of newer computer hardware. To use it, your motherboard needs to be compatible with UEFI mode.
Also, the UEFI BIOS usually runs on 64-bit systems, so you'll need CSM BIOS mode to boot 32-bit systems. For instance, Windows versions older than Vista SP1 (Windows XP, Windows 2000, Windows 95, etc.) don't work with UEFI mode.
UEFI mode also supports native multi-boot, so you can install more than one operating system on a single hard drive. You can then choose which one to boot from within the UEFI boot menu.
UEFI clearly wins when it comes to interface and overall user experience.
This is because CSM is a Character User Interface (CUI), meaning it works only with keyboard input. It's also not as intuitive and feature-rich as a UEFI interface.
UEFI, on the other hand, is a Graphical User Interface system (GUI) that looks more refined and is simpler to use. It adds support for mouse input, shortcut keys, scrollbars, icons, etc.
The CSM BIOS Mode uses MBR (Master Boot Record) disk partition table format, while the UEFI BIOS mode uses GPT (GUID Partition Table).
The difference between these disk partition formats is that MBR only allows 4 primary partitions on your hard disk and supports drives up to 2TB. In contrast, GPT format paired with UEFI BIOS supports 128 partitions and drives going up to 9ZB.
Also note that some operating systems like Windows require you to boot MBR disks exclusively with legacy/CSM BIOS mode and GPT disks with UEFI BIOS boot mode.
This isn't an issue with Linux systems with GRUB bootloader, where you can boot MBR disks in UEFI mode and GPT disks in legacy mode.
UEFI boot mode can be almost twice as fast as Legacy CSM boot mode because of "Fast Boot" support. It minimizes the number of devices required to begin the boot process.
In addition to that, UEFI boots almost instantly from power-saving modes like sleep, reboot, and hibernation. It contains features that bypass the power-on self-test (POST) process, which slows down a CSM boot system.
Not that these speed differences only apply to the boot time and not the general OS performance.
CSM is based on the traditional BIOS mode, which means it loses out on some of the security features of the updated UEFI mode.
On the other hand, UEFI offers all the modern security features, such as secure boot modes and disk encryption.
Depending on your needs, you may or may not want to use secure boot. However, having an additional security check at the firmware level can be beneficial against rootkits and ransomware.
The only scenario where UEFI BIOS can have bugs and flaws is when the operating system tries to load a wrong driver or remove the firmware configuration.
There's no definite answer to this question. Both CSM and UEFI have their place depending on your needs and requirements. But here's the take:
Use CSM BIOS booting mode if you:
Use UEFI BIOS mode if you:
CSM boot mode is being slowly phased out and replaced by UEFI BIOS mode due to the latter's enhanced security and better compatibility with newer machines.
As mentioned above, only use CSM BIOS mode if you want to use old legacy software or hardware that isn't compatible with UEFI mode. For most users today, you want to stick to UEFI as your default firmware.
The exact method to enable CSM mode is different for different motherboard brands. Generally, you'll find the setting by accessing the BIOS settings while booting up.
In most cases, you'll see an option called "Launch CSM" or "CSM Support" under the "Boot" tab. All you need to do is enable or disable the CSM boot system, then save and exit the UEFI booting screen.
Once enabled, your PC will automatically reboot with the desired legacy mode. If you can't find the setting, your motherboard likely doesn't support CSM.
Here's how to check which of the two boot modes your PC is booting through.
Tap on the Windows Search icon on your computer (or press the Windows key on your keyboard). Now type "System Information" and hit enter.
Find the entry that says BIOS Mode and check the value mentioned next to it. It will either say UEFI or Legacy.
Use CSM mode only if you want to boot an old 32-bit operating system on a 64-bit machine, or if you're booting from an old hard disk with MBR disk partitioning. Other than that, it's recommended to keep CSM legacy boot disabled.
Enabling CSM mode won't affect the general performance of your operating system. However, it might slightly increase the boot time because of the POST process and lack of Fast boot.
Hence, unless you really care about a few extra seconds spent during the boot process, it hardly makes any difference.
Windows 11 won't work with CSM enabled. Your computer should support secure boot to install Windows 11 which means it'll run only on UEFI BIOS mode.
However, you can still use Windows 11 with Secure Boot disabled, however, Microsoft doesn't recommend it since it can cause stability issues and lack of Windows updates.
No. Secure Boot is a feature available in the Unified Extensible Firmware Interface, or UEFI mode. However, it doesn't work with CSM enabled, or any legacy BIOS environment in general.
Since newer Windows OSes like Windows 8, 10 and 11 require you to have Secure Boot available, you can't use them with CSM or Legacy BIOS systems.
I hope you've learned everything there is to know about the CSM and UEFI system and the difference between the two boot modes.
To reiterate, CSM is nothing but a UEFI configuration that emulates legacy BIOS systems and offers backward compatibility. It's useful if you want to run old operating systems or storage devices that don't work on the UEFI system.
For most newer operating systems and computers, UEFI mode is the way to go. It's generally faster, more functional, and offers better security.