address
	LXI	H,12		;set to point downstream
	DAD	D		;now pointing to extent byte of directory
	MOV	C,M		;pull this byte in
	LXI	H,15		;ready to move down again
	DAD	D		;now point to 
	MOV	B,M		;put the extent byte back here
	POP	H		;recover the FCB