ExectOS Operating System | ExectOS Expands Beyond UEFI: Bringing BIOS Support

ExectOS Expands Beyond UEFI: Bringing BIOS Support

When we first started building ExectOS, we envisioned it as an operating system designed exclusively for UEFI-based systems. Over time, however, we realized that this requirement was an artificial limitation. Today, we are excited to announce that we have begun taking concrete steps toward supporting traditional PC/AT hardware with classic BIOS - broadening the reach of ExectOS to a much wider range of systems.

First Steps Toward BIOS Compatibility

We have already implemented an MBR boot sector and updated our CMake configuration to support BIOS-based emulation. As of now, ExectOS can be launched in several new testing configurations:

bochsvm: Bochs with BIOS
testkvm/testtcg: QEMU with classic BIOS (with KVM/WHPX acceleration or TCG without acceleration)
testefikvm/testefitcg: QEMU with modern UEFI (with or without acceleration), as before

This means developers can now experiment with ExectOS under both UEFI and BIOS environments using familiar tools like Bochs and QEMU.

A Unified Boot Partition Model

To simplify BIOS support, we are introducing a dedicated BOOT partition - similar to the recommended practice on Unix-like systems and analogous to the ESP (EFI System Partition) used by UEFI. Our bootloader will exclusively support FAT32 on this partition. It will house the bootloader itself, its configuration, and all modules. In other words, we are effectively bringing the concept of the ESP from the UEFI world into the BIOS realm.

What’s Next?

Full BIOS support is still some way off. The current MBR implementation and CMake adjustments are just the first step. We still need to:

Implement a VBR (Volume Boot Record) capable of locating and loading files from disk
Develop a preloader to prepare the environment for the main bootloader (enabling paging, long mode, etc.)
Significantly refactor XTLDR, to eliminate assumptions about UEFI, such as memory allocation APIs
Ensure all modules are environment-agnostic, running seamlessly on both BIOS and UEFI systems
Move PE/COFF file handling from the module level into the main XTLDR code

While there is still a long road ahead, this change opens up new possibilities for ExectOS. By removing the UEFI-only limitation, we are making the system more flexible and accessible.