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28

Category: DOS kernel
Flags: callout or callback (usually hooked rather than called)

INT 28 - DOS 2+ - DOS IDLE INTERRUPT

	SS:SP = top of MS-DOS stack for I/O functions
Return: all registers preserved
Desc:	This interrupt is invoked each time one of the DOS character input
	  functions loops while waiting for input.  Since a DOS call is in
	  progress even though DOS is actually idle during such input waits,
	  hooking this function is necessary to allow a TSR to perform DOS
	  calls while the foreground program is waiting for user input.	 The
	  INT 28h handler may invoke any INT 21h function except functions
	  00h through 0Ch.
Notes:	under DOS 2.x, the critical error flag (the byte immediately after the
	  InDOS flag) must be set in order to call DOS functions 50h/51h from
	  the INT 28h handler without destroying the DOS stacks.
	calls to INT 21/AH=3Fh,40h from within an INT 28 handler may not use a
	  handle which refers to CON
	at the time of the call, the InDOS flag (see INT 21/AH=34h) is normally
	  set to 01h; if larger, DOS is truly busy and should not be reentered
	the default handler is an IRET instruction
	supported in OS/2 compatibility box
	the _MS-DOS_Programmer's_Reference_ for DOS 5.0 incorrectly documents
	  this interrupt as superseded
	the performance of NetWare Lite servers (and probably other peer-to-
	  peer networks) can be dramatically improved by calling INT 28
	  frequently from an application's idle loop
SeeAlso: INT 21/AH=34h,INT 2A/AH=84h,INT 2F/AX=1680h

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  Copyright 2000   by Ralf Brown     Updated Jul 2000