39

This PDP-X Technical Memorandum #39, dated January 26, 1968, by Bev Young, details the interfaces for the Magnetic Tape (Magtape) Input/Output Processor (IOP). It describes how the Magtape IOP communicates within the PDP-X system, focusing on its interaction with the Arithmetic Processor (AP) and the tape transport devices.

A core feature of the PDP-X design is that peripheral device controllers (IOPs) are specialized processor units with read-only Control Memory, distinct from the Arithmetic Processor (AP). Their role is to execute micro-code sequences in response to AP commands, managing device operations (start, stop, data transfer) and reporting status.

Communication between any PDP-X processor (AP or IOP) and the rest of the system occurs via Main Memory (Memory Bus) and other devices (IO Bus). On the IO Bus, three specific mechanisms facilitate control and status exchange:

1. Inter-Processor Buffer (IPB): A register used for transmitting basic control information between the AP and the IOP, and for the IOP to signal the AP (e.g., job completion or errors via INTERRUPT signals).
2. IOP Status Word: Located in the IOP's Console Indicator Register, this provides the AP with comprehensive status about the IOP itself and the currently selected device, updated upon function initiation, chaining, termination, or error detection.
3. Device Status Register (Magtape Transport SR): Used to control and sense the operating state of the individual tape transport unit.

For Magtape operations, the IOP fetches detailed instructions from "Command Lists" stored in Main Memory. A specific memory location (address 540 for Magtape) points to the active Command List. Each Command List comprises a Command Word (CW) specifying the tape function (e.g., read, write, rewind), a Byte Address (BA) for memory data location, and a Byte Counter (BC) for data length. The system supports Data Linking (for transferring multiple data blocks) and Command Chaining (for executing a sequence of Command Lists) to automate complex operations.

The document provides detailed bit-level definitions for:

• Magtape Transport Status Register (SR): Outlines control bits (e.g., move direction, clock selection, write enable) and status bits (e.g., 9-track selection, transport at rest, rewind in progress, end-of-tape/beginning-of-tape detected) for the physical tape unit.
• Magtape IOP Status Word (SW): Defines error conditions (e.g., data overrun, bad tape, parity errors, read/compare discrepancies), transport readiness, and event detections (e.g., End-Of-File mark, Beginning-Of-Tape) reported by the IOP to the AP.
• A table of Program Command Word Function Codes is also included, detailing the specific actions the Magtape IOP performs based on bits 4-7 of the Command Word.

Overall, the memorandum emphasizes the granular control and detailed status reporting mechanisms crucial for efficient and reliable magnetic tape operations within the PDP-X architecture.