INT 66 - DIGPAK - SET DMAsee Direct Memory Access BACKFILL MODE
	AX = 069Ch
	DX = mode
	    0000h turn backfill mode on
	    0001h turn backfill mode off
Return: AX = status
	    0000h command ignored
	    0001h backfill mode set
SeeAlso: AX=069Dh,AX=069Eh