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  Interrupt:   15h     Function:   89h
 
  Title:       Enter Protected Mode
 
  Description:                                               [AT] [PS/2]
 
     Switches the CPU from real mode into protected mode.
 
     Input                                    Output
 
     AH = 89h                                 If function successful
     BH = Interrupt number for IRQ0,          (CPU is in protected mode)
          written to ICW2 of 8259 PIC #1        Carry flag: clear
          (must be evenly divisible by 8,       AH = 00h
          determines IRQ0-IRQ7)                 CS = User-defined selector
     BL = Interrupt number for IRQ8,            DS = User-defined selector
          written to ICW2 of 8259 PIC #2        ES = User-defined selector
          (must be evenly divisible by 8,       SS = User-defined selector
          determines IRQ8-IRQ15)
     ES:SI = segment:offset of Global         If function unsuccessful
             Descriptor Table (GDT)           (CPU is in real mode)
                                                Carry flag: set
                                                AH = FFh
                                    -♦-