Summary

Intel produced a draft specification of a 64-bit mode-only architecture, called X86S, that embodies the ideas outlined in the following white paper. Intel published the specification for the ecosystem to evaluate potential impacts to software. Based on inputs from the ecosystem, Intel has chosen not to pursue X86S. Intel will continue to maintain its longstanding commitment to software compatibility.

Introduction

The long life of Intel® architecture has resulted in a rich software ecosystem with an enormous installed base that extends from PCs to the cloud to mobile and from embedded devices to supercomputers and beyond.

Since its introduction over 20 years ago, the Intel® 64 architecture has become the dominant operating mode. As an example of this evolution, Microsoft stopped shipping the 32-bit version of their Windows 11 operating system. Intel firmware no longer supports non-UEFI64 operating systems natively. Today, 64-bit operating systems are the de facto standard. They retain the ability to run 32-bit applications but have stopped supporting 16-bit applications natively.

Cognizant of this evolution, Intel decided to explore opportunities for simplification in our hardware and software ecosystem.

Certain legacy modes have little utility in modern operating systems besides bootstrapping the CPU into 64-bit mode. It is worth asking the question, "Could these seldom-used elements of the architecture be removed to simplify a 64-bit mode-only architecture?"

Figure 1. The architecture proposed in this white paper would complete the transition to a 64-bit architecture, removing some legacy modes.

This white paper details the architectural enhancements and modifications that Intel investigated for a 64-bit mode-only architecture referred to as X86S (for simplification). Intel published this paper (and a draft specification) to solicit feedback from the ecosystem while exploring the benefits of extending the Instruction Set Architecture (ISA) transition to a 64-bit mode-only solution.

How Would a 64-Bit Mode-Only Architecture Work?

Intel 64 architecture designs come out of reset in the same state as the original 8086 and require a series of code transitions to enter 64-bit mode. Once running, these modes are not used in modern applications or operating systems.

Figure 2. Supported and unsupported modes in X86S

An exclusively 64-bit mode architecture would require 64-bit equivalents of technologies that currently run in either real mode or protected mode. For example:

• Booting CPUs (SIPI) starts in real-address mode today and would need a 64-bit replacement. A direct 64-bit reset state could eliminate the several stages of trampoline code to enter 64-bit operation.
• Today, using 5-level pages requires disabling paging, which requires going back to unpaged legacy mode. In the proposed architecture, it would be possible to switch to 5-level paging without leaving a paged mode.

These modifications could be implemented with straightforward enhancements to the system architecture affecting the operating system only.

What Would Be the Benefits of a 64-bit Mode-Only Architecture?

A 64-bit mode-only architecture removes some older appendages of the architecture, reducing the overall complexity of the software and hardware architecture. By exploring a 64-bit mode-only architecture, other changes that are aligned with modern software deployment could be made. The following changes were considered:

• Using the simplified segmentation model of 64-bit for segmentation support for 32-bit applications, matching what modern operating systems already use.
• Removing rings 1 and 2 (which are unused by modern software) and obsolete segmentation features like gates.
• Removing 16-bit addressing support.
• Eliminating support for ring 3 I/O port accesses.
• Eliminating string port I/O, which supported an obsolete CPU-driven I/O model.
• Limiting local interrupt controller (APIC) use to X2APIC and removing legacy 8259 support.
• Removing some unused operating system mode bits.

Legacy Operating Systems on 64-Bit Mode-Only Architecture

While running a legacy 64-bit operating system on top of a 64-bit mode-only architecture CPU may not be an explicit goal, the Intel architecture software ecosystem has sufficiently matured with virtualization products so that a virtualization-based software solution could use virtualization hardware (VMX) to deliver a solution to emulate features required to boot legacy operating systems.

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