INT 15 - SYSTEM - SWITCH TO PROTECTED MODE
        AH = 89h
        BL = interrupt number of IRQ0 (IRQ1-7 use next 7 interrupts)
        BH = interrupt number of IRQ8 (IRQ9-F use next 7 interrupts)
        ES:SI -> GDT for protected mode
               offset 0h  null descriptor (initialize to zeros)
                      8h  GDT descriptor
                     10h  IDT descriptor
                     18h  DS
                     20h  ES
                     28h  SS
                     30h  CS
                     38h  uninitialized, used to build descriptor for BIOS CS
Return: CF set on error
           AH = FFh  error enabling address line 20
        CF clear if successful
           AH = 00h
           in protected mode at specified address
Notes:  BL and BH must be multiples of 8
        the protected-mode CS must reference the same memory as the CS this
          function is called from because execution continues with the address
          following the interrupt call
SeeAlso: AH=87h,AH=88h,INT 67/AX=DE0Ch

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