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a confirming interconsole message to be sent back to the attach command. The command can relay to the user the tape mounting information by repeating the input information and concluding with "READY."

Dataphones

In addition to Teletypes, the IBM 7750 Communication Channel allows the attachment of higher speed devices (1200 bits/sec.) by means of Bell System Model 202B Dataphone sets, one at the 7750 and another at the device, with a telephone line between them. The devices (e.g., a PDP-1 computer or a 1620 computer) will appear as input-output units, and a user, from his console, may request to have them logically attached to his program. For convenience each device can be considered by the programmer to consist of several logical subunits; in particular, for control purposes one of these subunits (e.g., the typewriter keyboard) can represent a user console, so that an auxiliary console is not needed. Provision has been made for error detection and transmission correction in the internal character and message formats, which are further described in memo CC-210.

Command Programming

Whenever a user issues a command at his console, the command name and parameter segments are placed in order in a command parameter list, and the corresponding command program is located and started wherever indicated by the initial machine conditions of the program. The command program reads the parameter segments by issuing supervisor calls.

The only exception to the above process occurs with the resume command where the format for resuming program ${\textstyle \alpha }$, which has been "saved" by the save command, is $$\mathrm {resume} \ \alpha \ p_{1}\ p_{2}\ \ldots \ p_{n}$$. In this case, just before starting the ${\textstyle \alpha }$ program, the command list is reordered to contain the sequence ${\textstyle \alpha \ p_{1}\ p_{2}\ \ldots \ p_{n}}$. The purpose of this exception is to allow the user a trivial way to develop and use private commands which can also be compatibly added to the general system.

A useful property of the command mechanism is the ability of one command to call another command. The setting of the next command name

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