the in-out channel has a
word for, or needs a word from, the memory. When necessary, a memory
cycle is taken to serve the channel. The operation is initiated by an
in-out command. The in-out transfer command indicates the nature of the
transfer. The left half of the In-Out Register must contain the
starting address of the transfer, and the right half must contain the
number of words to be transferred. If the Sequence Break System is
connected, the completion of the transfer will signal the proper
sequence. If no Sequence Break System is connected, the completion of
the in-out channel transfer sets a program flag.

MAGNETIC TAPE

The system consists of tape units connected to the PDP-3 through a tape
control (TC). This tape is read or written in IBM 729I format. Two
hundred characters, each having 6 bits plus a parity bit, are written on
each inch of tape and the tape moves at 75 inches/sec. The tape control
has the job of connecting a specific unit to the PDP-3 and is a switch.
It also has the function of controlling the format of information that
is read or written on tape. In-out class commands instruct TC to the
type of information transfer and select the tape unit. Another IOT
command synchronizes the transfer of information through the TC to the
computer.

The IOT order to select the unit and function is decoded as follows: 1)
Three bits specify the function of TC. 2) The remaining 6 bits select
the unit.

_IOT Motion Commands for Magnetic Tape Units_

_IOT Code_ _Abbreviation_ _Function_

73....nn 60 mrb Read a binary record.
73....nn 61 mra Read an alphanumeric (BCD) record.
73....nn 62 mbb Backspace a binary record.
73....nn 63 mba Backspace an alphanumeric record.
73....nn 64 mwb Write a binary record.
73....nn 65 mwa Write an alphanumeric record.
73....nn 66 mlp Move tape to lead point (rewind).

Where the octal digits, nn, specify the unit number.

The motion commands have the deferred bit, thus, the program halts. If
the TC is free, the command will be transferred to the tape control for
action and the program restarts immediately. If the tape control is
currently busy with an instruction, i.e., it hasn't finished a previous
command, the motion command is held up until TC is free to execute the
new command.

The transfer of information from the computer to the TC is accomplished