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The bootloader cannot return back to BIOS. This allows UEFI firmware to be extended in a vendor-neutral way.Ī legacy BIOS loads a 512 byte flat binary blob from the MBR of the boot device into memory at physical address 7C00 and jumps to it. The A20 gate is enabled as well.Īdditionally, the platform initialization procedure of UEFI firmware is standardized. UEFI firmware performs those same steps, but also prepares a protected mode environment with flat segmentation and for x86-64 CPUs, a long mode environment with identity-mapped paging. The bootloader must enable the A20 gate, configure a GDT and an IDT, switch to protected mode, and for x86-64 CPUs, configure paging and switch to long mode. On a legacy system, BIOS performs all the usual platform initialization (memory controller configuration, PCI bus configuration and BAR mapping, graphics card initialization, etc.), but then drops into a backwards-compatible real mode environment. The differences between legacy BIOS firmware and UEFI BIOS firmware are where they find that code, how they prepare the system before jumping to it, and what convenience functions they provide for the code to call while running. In reality, both legacy motherboards and UEFI-based motherboards come with BIOS ROMs, which contain firmware that performs the initial power-on configuration of the system before loading some third-party code into memory and jumping to it.

Use the OVMF-blobs repo or download the RPM version, then use 7-Zip File Manager to extract the OVMF.fd file from the downloaded archive.Ī common misconception is that UEFI is a replacement for BIOS.

Under Linux, you can also install these with your distro's package manager, for example: RPM packages (official TianoCore packages).OVMF-blobs (unofficial precompiled 32 bit and 64 bit versions, easy to use).It might be tricky to find, so here are some alternative download links too: Otherwise for emulation and virtual machines, you'll need an OVMF.fd firmware image. You just have to enable it in the VM's preferences by clicking "Settings" / "Systems" / "Enable EFI (special OSes only)" checkbox. If you choose VirtualBox for virtualization, then UEFI is already included, no need to download the image manually. 7.2 My bootloader hangs if I use user defined EFI_MEMORY_TYPE values.7.1 My UEFI application hangs/resets after about 5 minutes.