Magtape IO Processor Interfaces

This document is a technical memorandum dated January 26, 1968, describing the interface and communication processes for a magnetic tape Input-Output Processor (IOP) within the PDP-X system.

The memo covers the following key areas:

• System Organization: Defines how the IOP communicates with the Arithmetic Processor (AP) and the rest of the system using an Inter-Processor Buffer (IPB), an IOP Status Register, and a Device Status Register.
• Inter-Processor Buffer (IPB): Explains how control information is passed between the AP and the IOP, specifically addressing command loading, interrupt signaling, and the sequence for initiating data transfers.
• Command Lists: Details the structure of command lists in main memory, which provide the IOP with the necessary instructions (e.g., tape function, byte address, byte count) to operate a tape transport.
• Status Registers: Provides a detailed breakdown of the bits within the Magtape Status Register (used for controlling tape transport state) and the Magtape IOP Status Word (used for reporting the operational state and errors to the programmer).
• Instruction Set: Lists the specific function codes and meanings for operations such as Rewind, Read, Write, and Image Mode.