INT 2F U - DOS 3+ internal - FIND UNREFERENCED DISK BUFFER
AX = 1210h
DS:DI -> first disk buffer to check
Return: ZF clear if found
DS:DI -> first unreferenced disk buffer
ZF set if not found

See also: 2F120E

2F1210 - DOS 3+ internal - FIND UNREFERENCED DISK BUFFER