with the pause and skip command, MPS or IOT 70. This command has the
deferred bit and halts a program until the TC can handle the transfer.
On completion, the transfer occurs and the program restarts. This is
used exclusively to synchronize the flow of information between a tape
unit and the computer. This command normally skips the following
instruction. If a flag is set in the TC, indicating incorrect
information flow, the skip does not take place.

The TC contains a 36 bit buffer which holds a complete word while
information is read or written. When an MPS order is given and the unit
is reading, the TC buffer is read into the IO. The MPS order given
during writing causes the IO to be transferred to the TC buffer.

Various conditions occurring in the TC cause the no-skip condition, when
an MPS is given. Tape control flags are examined by the command, examine
and clear flags, MEC or IOT 71. When MEC is given, the flags are put
into the IO for program interrogation, and the flags cleared. The flags
are: parity, end of tape, an end of record flag, and reading-writing
check.

The parity flag is set if the parity condition is not met while the tape
is being read (during MWA, MWB, MRA, or MRB).

The end of tape flag is set when the tape comes to the end of tape,
moving in either direction.

Three conditions set the read-write check flag: 1) If TC is inactive,
i.e., no unit or function selected, and an MPS instruction is given. The
MPS becomes a no-operation, no-halt instruction. 2) When reading
information and not emptying the TC buffer, by giving an MPS before more
information arrives from tape. 3) A unit becomes unavailable during a
normal sequence.

The end of record flag is set during reading or backspacing when the
tape comes to an end of record gap.

_Writing a Record of Information_

Information is written on the tape by giving a MWB or MWA command. This
sets a write binary or a write alphanumeric into the TC and selects the
unit. A motion select command is executed immediately if the TC is free,
otherwise, the command waits until it can be executed. Normal
programming can continue after the MWA or MWB is given for approximately
5 milliseconds. At this time, an MPS order is given and the program
pauses until information can be written. When the MPS is restarted,
information is transferred to the TC buffer from the IO. If no flags
have been set, the following instruction is skipped.

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