MOP

This document is a functional specification for the Maintenance Operation Protocol (MOP), Version 2.0, released in March 1978 as part of DECnet (Digital Network Architecture) by Digital Equipment Corporation.

MOP is a set of messages and rules designed for the maintenance of computer systems connected via a data link. Its primary functions include:

1. Loading (down-line loading) computer memory.
2. Dumping the contents of computer memory (e.g., for diagnostics after a failure).
3. Testing the communication link and its hardware components.
4. Forcing entry into MOP mode for unattended system control.
5. Transferring control to programs loaded in memory.

MOP operates between two directly connected computer systems: a Host (which initiates operations, provides data for loads, and receives data from dumps) and a Satellite (the target for loads, source for dumps, and echoer for link tests). It runs in an alternate half-duplex mode over a single data link.

A key design principle is that MOP is built to require a minimum amount of code, allowing basic functions to reside in read-only memory (ROM) on the satellite for bootstrapping purposes. While MOP is independent of specific data link control procedures, within DECnet, it typically uses the maintenance mode of the Digital Data Communications Message Protocol (DDCMP) for essential services like message framing and bit error detection. Error recovery at the MOP level is primarily handled through timeouts and retransmissions. The document details message formats and operational states for both host and satellite systems.