INT 21 - DOS 3.0+ internal - GET ADDRESS OF DOS SWAPPABLE DATA AREA

	AX = 5D06h
Return: CF set on error
	   AX = error code (see #01680)
	CF clear if successful
	    DS:SI -> nonreentrant data area (includes all three DOS stacks)
		(critical error flag is first byte) (see #01687)
	    CX = size in bytes of area which must be swapped while in DOS
	    DX = size in bytes of area which must always be swapped
Notes:	the Critical Error flag is used in conjunction with the InDOS flag
	  (see AH=34h) to determine when it is safe to enter DOS from a TSR
	setting CritErr flag allows use of functions 50h/51h from INT 28h under
	  DOS 2.x by forcing use of correct stack
	swapping the data area allows reentering DOS unless DOS is in a
	  critical section delimited by INT 2A/AH=80h and INT 2A/AH=81h,82h
	under DOS 4.0, AX=5D0Bh should be used instead of this function
	SHARE and other DOS utilities consult the byte at offset 04h in the
	  DOS data segment (see INT 2F/AX=1203h) to determine the SDA format
	  in use: 00h = DOS 3.x, 01h = DOS 4.0-6.0, other = error.
	DR DOS 3.41+ supports this function, but the SDA format beyond the
	  first 18h bytes is completely different from MS-DOS
BUG:	calling this function with certain values in DX crashes Novell DOS 7.0
	  prior to Update 14
SeeAlso: AX=5D0Bh,INT 2A/AH=80h,INT 2A/AH=81h,INT 2A/AH=82h