Hello everyone!
I write an OS in my free time, and decided to share the result here.
The capabilities of BIOS are used (not uefi), direct output to video memory 0xB800 in 80x25/16 mode, menu selection with keys 1-7. In principle, nothing new - the bootloader reads the kernel at address 0x0600, leaving the stack in the same place 0x7C00.

In addition to the legacy ATA interface, there is support for modern SATA in AHCI mode. Later, I plan to add initialization of the NVM controller on the PCI bus, as well as NVMe on the PCI-Express bus. The software model of AHCI, NVM, NVMe is different, so I will have to implement three different modules. Since their ports are mapped to MMIO memory, the real mode kernel code has a switch to "unreal" mode, with access to 4GB of memory through the FS register.

At the moment I am working on the x64 entry with the "Long" memory model - it is not finished yet, so the menu item (6) is empty for now. The archive contains the entire source code, as well as a 1.44 MB floppy image of "CodebyOS". To conduct tests on real hardware, you can manually create a bootable USB-Flash, and in the BOOT folder there is the source code of the USB-loader. Here is the result.