When a computer powers on, low-level firmware initializes the hardware before the operating system loads and prepares the system for booting. There are two forms of firmware: UEFI, which stands for Unified Extensible Firmware Interface, and the other form is BIOS, which stands for Basic Input Output System. Both UEFI and BIOS allow the hardware to communicate and boot up the operating system.
With the usage of UEFI in modern systems, understanding the differences between UEFI and BIOS is important. It makes decisions related to changes to settings and hardware compatibility. This guide provides a detailed technical breakdown of UEFI and BIOS, explaining their architectures, boot processes, limitations, and real-world implications.
How UEFI Works: The Boot Process
How BIOS Works: The Boot Process
Key Advantages of UEFI Over BIOS
What is BIOS?
BIOS is a firmware interface that dates back to the early days of PCs (IBM developed it in 1975). It’s the standard firmware that has been used in computers for a long time. It is low-level software embedded in a chip that is located on the motherboard that starts the hardware components and transfers control to the operating system. It resides on a chip on the motherboard and is responsible for:
- Hardware initialization (CPU, RAM, storage, peripherals).
- System configuration (via CMOS/EEPROM settings).
- Booting the operating system.
What is UEFI?
UEFI replaced BIOS in the early 2000s. It’s a more recent alternative to BIOS, created to address its shortcomings and improve the performance of the system. It is not just an update but a complete redesign with the following:
- Modular architecture (supports drivers and extensions).
- Advanced security features (Secure Boot, TPM integration).
- Faster and more flexible booting.
UEFI vs BIOS
The differences between UEFI and BIOS are in their architecture, booting process, the storage that is being used, and security features. Older firmware, BIOS, that works in a 16-bit mode, has a slower booting process and uses MBR partitioning.
On the other hand, with UEFI, there is support for both 32-bit and 64-bit functioning, along with faster booting. It is due to advanced startup techniques, enlarged storage devices, and GPT partitioning; alongside these features. In addition, UEFI safeguards systems with Secure Boot, ensuring that malware is reduced by only allowing approved software to be run.
Storage Limitations
| Scenario | BIOS (MBR) | UEFI (GPT) |
| Max Disk Size | 2 TB | 9.4 ZB (practically unlimited) |
| Partitions | 4 primary (or 3+1 extended) | 128 Primary |
| Boot Drive | Must be MBR | Must be GPT |
Example: If you install a 4 TB HDD in BIOS mode, only 2 TB will be usable.
OS Compatibility
| OS | BIOS(Legacy) | UEFI |
| Windows 7 | Yes (with CSM) | Yes (64-bit) |
| Windows 10/11 | No (deprecated) | Required |
| Linux (Most Distros) | Yes | Yes (better support) |
| macOS | No | Required |
How UEFI Works: The Boot Process
1. Pre-EFI Initialization (PEI)
This process initializes the CPU memory and other components of the motherboard.
2. Driver Execution Environment (DXE)
Loads UEFI drivers for storage, networking, and graphics components.
3. Boot Manager
Reads NVRAM to locate bootloaders (no reliance on MBR).
4. GPT Partitioning
Uses GUID Partition Table (GPT), supporting:
- Disks >2 TB (theoretical max: 9.4 ZB).
- Up to 128 partitions per disk.
5. Secure Boot (Optional)
Verifies bootloader signatures to prevent malware execution.
6. OS Loading
Directly executes the OS kernel in 64-bit mode.
How BIOS Works: The Boot Process
1. Power-On Self-Test (POST)
Checks hardware integrity (RAM, CPU, storage). If errors occur, it emits beep codes or displays error messages.
2. Hardware Initialization
Detects and configures storage devices, USB, and other peripherals.
3. Boot Device Selection
Searches for a bootable device in the order specified in BIOS settings.
4. MBR (Master Boot Record) Loading
Reads the first 512 bytes of the disk (MBR), which contains:
- Bootloader (e.g., GRUB, Windows Boot Manager).
- Partition table (limited to 4 primary partitions).
5. OS Handoff
The bootloader loads the OS kernel.
Tip: When is BIOS Still Used?
- Legacy systems (Windows XP, older Linux distros).
- Some industrial/embedded systems where UEFI is unnecessary.
- Virtual machines are configured for backward compatibility.
Key Advantages of UEFI Over BIOS
UEFI has several benefits over BIOS. It significantly improves boot speed by using an advanced initialization process, reducing system startup times. UEFI’s support for larger storage devices makes it more suitable for modern computing needs, eliminating the limitations imposed by BIOS. Security enhancements, e.g., cryptographic verification and Secure Boot, protect against boot-time malware attacks. Additionally, UEFI provides a more user-friendly interface with better accessibility features, including support for high-resolution displays and mouse navigation, simplifying system management.
| Feature | UEFI | BIOS |
| Boot Mode | 32/64-bit | 16-bit |
| Partition Scheme | GPT (9.4 ZB) | MBR (2 TB) |
| Boot Speed | Faster (parallel init) | Slower (sequential) |
| User Interface | Graphical (mouse support) | Text-only |
| Security | Secure Boot, TPM | None |
| Networking | Built-in PXE boot | Requires ROM |
| Driver Support | Modular (EFI drivers) | Limited to ROM |
UEFI-Only Features
- Secure Boot: Ensures only signed bootloaders run (blocks rootkits).
- Fast Boot: Skips unnecessary hardware checks.
- UEFI Shell: A command-line environment for troubleshooting.
- Network Booting: Supports PXE (useful for diskless workstations).
Convert from BIOS to UEFI?
- Backup data (converting MBR → GPT is destructive).
- Use gptgen (Linux) or mbr2gpt (Windows) to convert partitions.
- Reinstall the OS in UEFI mode (clean install recommended).
Warning: Some older motherboards support UEFI but lack Secure Boot (required for Windows 11).
Which One Should You Use?
Most modern computers come with UEFI by default due to its superior features and performance benefits. However, some legacy systems or older hardware may still require BIOS. Especially if they do not support UEFI if you are installing a new operating system or setting the system for optimal performance. In that case, UEFI is the recommended choice due to its enhanced boot speed, security, and compatibility with larger storage devices.
| Use Case (Recommended) | Firmware |
| Modern OS (Win 10/11, Linux) | UEFI (Faster, Secure Boot) |
| Older OS (Win 7, XP) | BIOS (Legacy Mode) |
| Large HDD (>2 TB) | UEFI (GPT Required) |
| Security (Enterprise, Win 11) | UEFI + Secure Boot |
| Legacy Hardware | BIOS (if UEFI unavailable) |
Conclusion
UEFI is a newer and more efficient version of the BIOS. With better performance, improved security features, and support for modern storage, UEFI has become the industry’s priority. UEFI is the latest modem replacement and improves in every possible way compared to the BIOS, which is the older form and many consider obsolete.
BIOS is an old framework class that identifies the legacy architecture. New PCs use UEFI, and if you switched from BIOS to UEFI, you had to reinstall the OS in GPT mode.
