+-----------------+
                   |     T.IRNG      |
                   +-----------------+
                   |     T.IPUT      |
                   +-----------------+
                   |     T.ICTR      |
                   +-----------------+
                   |     T.IGET      |
                   +-----------------+
                   |     T.ITOP      |
                   +-----------------+
                   |   input ring    |
                   | (default size = |
                   |    134 bytes)   |
                   +-----------------+
                   |     T.OPUT      |
                   +--------+--------+
                   | ^O FLG | T.OCTR |
                   +--------+--------+
                   |     T.OGET      |
                   +-----------------+
                   |     T.OTOP      |
                   +-----------------+
                   |   output ring   |
                   | (default size = |
                   |     40 bytes)   |
                   +-----------------+
                   |     T.RTRY      |
                   +-----------------+
                   |     T.TBLK      |
                   |    (7 words)    |
                   +-----------------+
                   |     T.AST       |
                   | (2 words when   |
                   |   mapped mon.)  |
                   +--------+--------+
                   | T.XCNT | T.XFLG |
                   +--------+--------+
                   |     T.XPRE      |
                   +-----------------+
                   |     T.XBUF      |
                   |    (3 words)    |
                   +-----------------+
                   |     T.CNT       |
                   +-----------------+