INT D4 - PC-MOS/386 v5.01 - GET TASK CONTROL BLOCK SEGMENT/SELECTOR             
	AH = 27h
	BX = task ID (FFFFh if current task) (see AH=19h)
Return: CF clear if successful
	    DX = segment/selector for the Task Control Block (see #04006)
	CF set on error
	    AX = error code (see #01680 at INT 21/AH=59h/BX=0000h)
Note:	this function supersedes AH=04h
SeeAlso: AH=26h,AH=28h,AH=29h,AH=2Ah