neon wrote:
For these scenarios you'll need to switch between real and protected modes or use unreal mode.
In addition to what Octo said, you really should not be using unreal mode. Unreal mode can be destroyed whenever BIOS switches to protected mode and then back to real mode. Indeed, BIOS has no way to tell that unreal mode is in use. And BIOS can switch modes potentially on every interrupt it handles, both hardware and software. There is a lot of hardware that cannot be serviced in real mode at all, e.g. USB and NVMe. SeaBIOS exists and you can look up its source code, and see all the mode switches in there.
On the flipside, function 15/87 is used in popular boot loaders, and therefore highly likely to be both present and working.