[commentereleven]

I think you're talking about unreal mode.

http://wiki.osdev.org/Unreal_Mode

[a1k0n]

That's right, I was, but on further reflection you don't need to do anything special in this case since it's just doing 32 bit math, not 32 bit addressing. EAX/EBX/etc should just work in real mode IIRC.