INT 21,36 - Get Disk Free Space

	AH = 36h
	DL = drive number (0=default, 1=A:)

	on return:
	AX = sectors per cluster
	   = FFFF if drive is invalid
	BX = number of available clusters
	CX = number of bytes per sector
	DX = number of clusters per drive

	- used to determine available space on specified disk
	- see	INT 21,1B   INT 21,1C