Your motherboard contains embedded firmware or low-level software. It reads and executes instructions on how to boot your computer. This firmware typically falls into two categories: Basic Input/Output System (BIOS), which is the older standard, and Unified Extensible Firmware Interface (UEFI), which is the modern version used in today’s motherboards.
If you’ve interacted with your motherboard’s firmware, you’ve likely noticed support for both the UEFI and Legacy BIOs modes. This is possible due to the Compatibility Support Module (CSM), which is a UEFI component that provides BIOS CSM Support so that your motherboard can emulate a BIOS environment.
This article is going to cover all questions relating to CSM vs. UEFI and provide an explainer of what a CSM is, when it should be used, and how to activate it.
What are Legacy BIOS and UEFI?
In order to get a proper grasp on what CSM is, we first need to understand exactly what is meant by the terms Legacy BIOS and UEFI. Both of these systems are what’s called a bootloader, a collection of software, programmed into the motherboard, that tells the computer what to do when it first turns on.
The process includes finding the operating system (OS), what parts of it to access initially, and what hardware is available to the computer. The significant difference between these systems is in how the OS is accessed, as this determines the capabilities of the entire computer.
Legacy BIOS uses something called the Master Boot Record (MBR), a small portion on the hard drive, resembling a partition, containing the startup protocols of the OS. Up until 2007, this was the main method used for startup systems on computers. However, advancements in technology began to push past the storage limits of the MBR, as it can only support any single partition on a hard drive being no more than 2.2 TB, forcing the need for something more powerful.
UEFI was introduced in the late 90s, but quickly gained ground once it became clear that it addressed the limitations of the MBR. All modern computers now use UEFI, as it is able to interact with modern hardware, and allows computers to run with storage capacities exceeding 2 TB, something that’s simply impossible when using an MBR.
It is accomplished by organizing data with a GUID Partition Table (GPT), which assigns unique identifiers to each bit of information and subsequently extends its theoretical storage support by over 9 billion TB. This is indeed powerful, but the main limitation of UEFI is that it’s designed for modern systems, so older operating systems can’t be run without aid, which is precisely where CSM comes into play.
What is CSM and How Does it Relate to UEFI?
The Compatibility Support Module (CSM), is often mistaken as the Legacy BIOS bootloader but is in fact, an integrated component of UEFI. As the name suggests, CSM is what allows a computer using UEFI to load an older OS that would normally require a Master Boot Record (MBR) in a Legacy BIOS system.
In other words, it allows you to run a Legacy BIOS OS, such as Windows Vista or any 32-bit OS, on a UEFI computer. This backward compatibility is accomplished by enabling UEFI to emulate a Legacy BIOS environment within it, which is then used to load the accompanying operating system.
As you can see, this makes it difficult to actually compare CSM vs. UEFI, as CSM is not a dedicated firmware module on its own, but rather a component of the Unified Extensible Firmware Interface (UEFI) that provides compatibility support for individuals that may want to run their computer in the Legacy BIOS boot mode.
With this in mind, instead of comparing CSM vs. UEFI, we’ll instead explore more of why, and how, to enable and use CSM on your computer.
How Does CSM Change the Bootloading Process?
As mentioned above, CSM allows UEFI to emulate a Legacy BIOS environment within it, allowing an older OS to be run on a newer system. Effectively, your computer will run as though it were a genuine Legacy BIOS system. That being the case, we can draw comparisons between different aspects of the 2 bootloaders in order to display their differences, as shown in the table below.
| Legacy BIOS | UEFI | |
| Hard Drive Storage Support | Up to 2.2 TB per partition | Up to 9 ZB per partition |
| Firmware Architecture | 32-bit speed | 64-bit speed |
| Partition Format | Master Boot Record (MBR) | GUID Partition Table (GPT) |
| Security Capability | No TPM* support | Secured TPM support |
| Boot Speed | Slower | Faster |
| Setting Accessibility | Via BIOS only | Via BIOS or OS |
*TPM (Trusted Platform Module) is a chip, installed on a motherboard that secures hardware via integrated cryptographic keys. It is used to defend against threats such as ransomware.
When to Enable CSM and Legacy BIOS
The number one time when CSM vs. UEFI should be enabled and is necessary to do so is when you’re using an older operating system like Windows Vista, Windows XP, Windows ME, Windows 2000, or Windows 95 or older.
If you were to choose to run a newer operating system in a 32-bit architecture, then you would also enable CSM and run it via the Legacy BIOS option. So, theoretically, you could run Windows 10 for example, in Legacy BIOS as a 32-bit system. However, it’s not recommended to do this as the 32-bit architecture runs slower due to its limited data pathways, and it means that programs that use 64-bit architecture will not work.
As a result, if you’re running an operating system that is UEFI compatible (ie, released after Windows Vista, or uses a 64-bit system), it should, ideally, run on that platform with the CSM disabled. Following this rule will ensure that your computer runs smoothly, free from any boot errors or incompatibility issues.
How to Know if CSM is Enabled
If you’re running a newer computer, particularly a pre-built one, it is highly likely that CSM has been disabled by default. However, you can verify this for yourself by following the steps below:
- Start by pressing the Windows Key on your keyboard. Type System Information into the search box, then press the Enter Key.
- In the window that opens, search the list for an entry labeled BIOS Mode and take note of what is written on its right. If it says UEFI, CSM will be disabled by default. If it says Legacy and you’re using a newer computer, CSM is most likely enabled.
How Do You Change Boot Mode From UEFI to CSM?
If you find yourself in a situation where you need to activate CSM vs. UEFI, like installing an old OS or trying to access an old hard drive with an old OS already installed, you can easily do so by following a few quick steps.
However, the location of the setting varies slightly, depending on the brand of your motherboard, so be sure to find that information in advance. We will cover the steps for Asus, ASRock, MSI, and Gigabyte motherboards below.
Note: All motherboards require that the computer is turned off to start with.
1. How to Change Boot Mode from UEFI to CSM in ASUS
- Begin by powering on your computer and repeatedly pressing either the DEL Key or the F2 Key on your keyboard if DEL is unavailable. This should boot your computer to its BIOS settings.
- Using the left and right arrow keys on your keyboard, navigate through the menus to the Boot Tab.
- Navigate to the CSM Option, then locate the Launch CSM setting and change it to Enabled.
- Finally, navigate to, and select, Save & Exit to restart your computer with the new setting.
2. How to Change Boot Mode from UEFI to CSM in ASRock
- Begin by powering on your computer and repeatedly pressing either the DEL Key or the F2 Key on your keyboard if DEL is unavailable. This should boot your computer to its BIOS settings.
- Using the left and right arrow keys on your keyboard, navigate through the menus to the Boot Tab.
- Navigate to the CSM Option, then change it to Enabled.
- Navigate to the Exit Tab and select Save and Exit to restart your computer with the new setting.
3. How to Change Boot Mode from UEFI to CSM in MSI
- Begin by powering on your computer and repeatedly pressing either the DEL Key or the F2 Key on your keyboard if DEL is unavailable. This should boot your computer to its BIOS settings.
- Using the left and right arrow keys on your keyboard, navigate through the menus to the Boot Tab.
- Select the option labeled Boot Mode Select. To enable CSM, choose Legacy Mode, to disable it, choose UEFI Only, and to have your computer automatically detect what setting to use, choose UEFI Plus Legacy Mode.
- Press the F10 Key on your keyboard to save the new setting and restart your computer.
4. How to Change Boot Mode from UEFI to CSM in Gigabyte
- Begin by powering on your computer and repeatedly pressing either the DEL Key or the F2 Key on your keyboard if DEL is unavailable. This should boot your computer to its BIOS settings.
- Using the left and right arrow keys on your keyboard, navigate through the menus to the Security Tab.
- Select the option labeled CSM Support by double-clicking on it with your mouse.
- Set CSM Support to be Enabled.
- Navigate to the Power Tab, then select Save & Exit Setup to restart your computer with the new setting.
CSM vs. UEFI Explained!
We hope that for those of you who are looking to install older operating systems, or just curious about CSM vs. UEFI boot modes, this comparison and explainer article has helped you learn all about the CSM component. It includes what situations it’s good for, when it should be enabled, how to check if it’s active, and how to enable it when necessary.
Just remember that if you use CSM, you must have an operating system installed that is compatible. Let us know in the comments below if this article helped you in any way.
