[AT]
Expects: AH    89H
BH    index into IDT defining access of IRQ 0-7
BL    index into IDT defining access of IRQ 8-15
ES:DI addr of a GDT built by the caller
                                                                  
Returns: AH    00 and CY=NC means successful and return ion protected mode
ff and/or CF=CY means no can do.
                                                                  
Info: This sets the processor into protected mode and passes control to
the code segment identified by the GDT provided by the caller.

There is no BIOS service to return to real mode.

This is tricky, unless you really know what you're doing; only
writers of operating systems need bother.  You better study the
BIOS listing on this one.  Better yet, use the DPMI API.

See Also: INT 15H (extended AT services)
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INT 15H 89H: Switch to Protected mode