Get protected mode interface

        Entry:
          AX = DE01h
          ES:DI -> 4K page table buffer
          DS:SI -> three descriptor table entries in GDT
                   first becomes code segment descriptor, other two for use by
                   main control program
        Return:
          AH = 00h successful
               DI -> first unused page table entry in buffer
               EBX -> protected mode entry point in code segment
          AH = nonzero  failed