Ever wonder what it's like to run without hardware protection? Want to have direct access to all attached peripherals and disks directly? Enter real mode, an IA32 specific feature introduced with the 80286. This actually wasn't a new feature at that point, since the 80186 and the 8086 also had real mode. Rather, the 80286 introduced the distinction of real mode and protected mode.
http://en.wikipedia.org/wiki/Real_mode
I found it funny that in IA32, you can go from protected mode to real mode by resetting and keeping the contents of RAM alive. What a hack!
With real mode, you can't (shouldn't) access non-8086 extensions because it's nice to be backwards compatible. That being said, you really only have the basic ops available to you in real mode. That means no MMX or SSE for your homebrew BIOS =(
Interesting stuff =)
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