When GRUB hands control to your kernel entry point via the Multiboot2 protocol, your CPU is in 32-bit Protected Mode . Interrupts are likely off. The A20 line is enabled. Segment registers have been set up with flat 4 GiB descriptors. You're running, but you're still in the 1990s.

To write a real Rust kernel — one that can use 64-bit pointers, access more than 4 GiB of RAM, and benefit from the x86_64 calling convention — we need to leave that world behind and enter Long Mode .

This transition is not handled by a library. There's no std . There's no runtime. There's nothing between you and the silicon except a handful of assembly instructions and a very specific sequence of CPU configuration steps. Get the sequence wrong, and you'll triple-fault into a reboot.…