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= Overview

The *A4X* firmware forms the contents of the boot ROM of XR/computer platforms.#footnote([See the _XR/computer Systems Handbook_ for more information.]) This is the first code that executes in the system after processor reset, and is responsible for initializing the integral devices and enabling the user to select a boot device.

It is designed to be very simple. It executes entirely out of ROM, using a small region in low memory for volatile data. Virtual addressing is disabled and polling is used for all I/O.

This document describes the user interface of the *A4X* firmware, and the fundamentals of its boot protocol, including an overview of the A3X partition table (APT) format.

Implementation details of the firmware are not described here; they are best defined by the source code of the firmware itself.