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Category: DOS extenders
Flags: available only in protected mode

INT 21 - Phar Lap 386/DOS-Extender v4.0+ - READ/WRITE IDT DESCRIPTOR

	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

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  Copyright 2000   by Ralf Brown     Updated Jul 2000