How Does Ventoy Handle UEFI Boot Compared to Legacy BIOS?

Ventoy provides a unified multiboot solution by formatting the USB drive into two distinct partitions that accommodate both legacy BIOS and modern UEFI architectures. When a system powers on, the computer's firmware dictates the execution path: legacy BIOS executes boot code directly from the drive's Master Boot Record (MBR), whereas UEFI locates and loads executable boot files from an EFI System Partition (ESP). Understanding these differences helps clarify how Ventoy initializes its environment, passes control to chosen operating system images, and manages hardware compatibility.

Dual-Partition Architecture

To support both modes seamlessly from a single flash drive, Ventoy divides the storage device into two partitions:

• Partition 1 (Data): Takes up the vast majority of the drive space, typically formatted as exFAT, NTFS, or ext4. This is where users place raw ISO, WIM, IMG, or VHD files.
• Partition 2 (VTOYEFI): A small, hidden FAT-formatted EFI System Partition placed at the end of the drive. It holds the necessary core binaries for GRUB2, EFI loaders (BOOTX64.EFI, BOOTIA32.EFI, etc.), and the Ventoy bootloader framework.

The Boot Process: Legacy BIOS vs. UEFI

The mechanism used to reach Ventoy's main menu varies based on the firmware mode running on the motherboard.

Legacy BIOS Mode

1. The BIOS reads and executes the boot code stored in the Master Boot Record (Sector 0) of the drive.
2. The initial boot code loads stage components from Partition 2 into memory to initialize the GRUB2 legacy environment.
3. Ventoy hooks into BIOS interrupt routines (specifically INT 13h). This virtualizes a CD/DVD drive in RAM pointing directly to the selected ISO on Partition 1.
4. When an ISO is selected, Ventoy hands over control to the image's legacy boot sector using standard BIOS call emulation.

UEFI Mode

1. The motherboard firmware parses the partition table and directly searches the EFI System Partition (Partition 2) for standard UEFI bootloaders.
2. The firmware loads the appropriate EFI executable (such as BOOTX64.EFI for 64-bit systems or BOOTIA32.EFI for 32-bit UEFI systems).
3. Ventoy installs a runtime EFI driver (specifically a Block I/O protocol hook) into the UEFI environment instead of relying on legacy interrupts.
4. When an image is launched, the ISO is presented as an emulated UEFI-compliant virtual disk. Ventoy then chains execution into the EFI bootloader contained within the ISO itself.

Key Differences Between the Two Modes

Secure Boot UEFI mode supports Secure Boot validation, which ensures that only cryptographically signed boot files run at startup. Ventoy incorporates a signed shim bootloader that allows it to operate with Secure Boot enabled (sometimes requiring the user to enroll Ventoy's MOK key once). Legacy BIOS lacks cryptographic signature checks entirely.

Hardware Access and Disk Addressing Legacy BIOS systems communicate through 16-bit real-mode services and may suffer from vendor-specific address limitations, such as failing to read past 137 GB on large storage drives. In contrast, UEFI operates in 32-bit or 64-bit protected/long mode natively, providing direct, unrestricted access to large drives formatted with GPT or MBR partition schemes.

Payload Compatibility Ventoy cannot convert the boot mode of an operating system image. If an ISO only contains legacy bootloaders, it will fail to boot if Ventoy was started in UEFI mode. Conversely, modern UEFI-only ISOs cannot be booted if Ventoy is running under a legacy BIOS session. The boot mode chosen by the host machine when starting Ventoy strictly determines the mode used for all downstream installations.