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INT 15 - SYSTEM - SWITCH TO VIRTUAL MODE (AT,XT286,PS50+)
        AH = 89h
        BL = interrupt number of IRQ0 (IRQ1-7 use next 7 interrupts)
        BH = interrupt number of IRQ8 (IRQ9-F use next 7 interrupts)
        DS:SI -> GDT for protected mode
               offset 0h  null descriptor
                      8h  GDT descriptor
                     10h  IDT descriptor
                     18h  DS
                     20h  ES
                     28h  SS
                     30h  CS
                     38h  uninitialized, used to build descriptor for BIOS CS
        CX = offset into protected-mode CS to jump to
Return: CF set on error
           AH = 0FFh  error enabling address line 20
        in protected mode at specified address if successful
SeeAlso: INT 67/AX=DE0Ch