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  Description:  Enter Protected Mode      [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)                If function unsuccessful
  ES:SI=segment:offset of Global            (CPU is in real mode)
                Descriptor Table (GDT)      Carry flag=set
                                            AH=FFH
                                    -♦-