Int 21 Fn 253D  P - Phar Lap 386/dos-extender V4.0+ - Read/write Idt Desc  [E]

   AX = 253Dh
   BL = interrupt number
   ECX = direction (0 = read, 1 = write)
   DS:EDX -> 8-byte buffer for descriptor

Return: CF clear if successful
       DS:EDX filled if reading
   CF set on error
       EAX = error code (0082h if running under DPMI)
Desc:  access hardware-level IDT rather than the internal 386/DOS-Extender
     shadow IDT

Notes: this call will always fail under DPMI because it is not possible to
     access the IDT
   the descriptor is not checked when writing
   this call can normally be used only by programs running in ring 0
     because the processor does not allow an interrupt to be vectored to
     a less privileged ring