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        Switch to protected mode (INT 15h, 89h)

        Entry:
          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 = pointer to 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