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2F13

Category: DOS kernel
Flags: Undocumented function

INT 2F - DOS 3.2+ - SET DISK INTERRUPT HANDLER

	AH = 13h
	DS:DX -> interrupt handler disk driver calls on read/write
	ES:BX = address to restore INT 13 to on system halt (exit from root
		 shell) or warm boot (INT 19)
Return: DS:DX set by previous invocation of this function
	ES:BX set by previous invocation of this function
Notes:	IO.SYS hooks INT 13 and inserts one or more filters ahead of the
	  original INT 13 handler.  The first is for disk change detection
	  on floppy drives, the second is for tracking formatting calls and
	  correcting DMA boundary errors, the third is for working around
	  problems in a particular version of IBM's ROM BIOS
	before the first call, ES:BX points at the original BIOS INT 13; DS:DX
	  also points there unless IO.SYS has installed a special filter for
	  hard disk reads (on systems with model byte FCh and BIOS date
	  "01/10/84" only), in which case it points at the special filter
	most DOS 3.2+ disk access is via the vector in DS:DX, although a few
	  functions are still invoked via an INT 13 instruction
	during Windows 3.1 startup this function seems to be used to
	  temporarily point DOS to a dummy handler in WDCTRL.386 which always
	  halts the system with a fatal error message.	If DS hasn't changed
	  on return from the function, Windows will display the error message
	  "Invalid DOS version".
	this is a dangerous security loophole for any virus-monitoring software
	  which does not trap this call ("INT13", "Nomenklatura", and many
	  Bulgarian viruses are known to use it to get the original ROM entry
	  point)
	the preloadable Novell DOS 7+ SECURITY.BIN driver $SECURE$ traps
	  this call.
BUG:	Novell DOS 7 IBMBIO.COM before 1995-05-08 trashed AX on return from
	  this function. VGACOPY by Thomas M”nkemeier's VGA Software GmbH
	  crashed due to this.	Later releases of Novell DOS 7 preserved the
	  contents of the AX register.
SeeAlso: INT 13/AH=01h,INT 19,INT 9D"VIRUS"

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