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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.
See Also: INT 13/AH=01h - INT 19 - INT 9D"VIRUS"
Category: DOS Kernel - Int 2Fh - D
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