DOS Internal - Read vtoc

        Entry:
          AX = 1505h
          ES:BX -> 2048-byte buffer
          CX = drive number (0=A:)
          DX = sector index (0=first volume descriptor,1=second,...)
        Return:
          CF set on error
            AX = error code (15=invalid drive,21=not ready)
          CF clear if successful
            AX = volume descriptor type (1=standard,FFh=terminator,0=other)