Intel looks to 64bit only processor architecture
Intel is proposing dropping support for 32bit modes in its processors and moving to a 64bit only architecture.
The company introduced 64bit mode back in 2004, and Microsoft dropped the 32bit modes in the Windows 11 operating system, and Intel firmware no longer supports non UEFI64 operating systems natively so 64bit operating systems are the de facto standard today. They retain the ability to run 32bit applications but have stopped supporting 16bit applications natively.
Intel believes there are opportunities for simplification in hardware and software and has published a white paper looking at the options for what it calls x86S: Intel’s x86s 64bit only proposal
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 64bit mode. Once running, these modes are not used in modern applications or operating systems.
An exclusively 64-bit mode architecture will require 64bit 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 needs a 64bit replacement. A direct 64bit reset state eliminates the several stages of trampoline code to enter 64bit operation.
Today, using 5-level pages requires disabling paging, which requires going back to unpaged legacy mode. In the proposed architecture, it is possible to switch to 5-level paging without leaving a paged mode.
A 64bit mode-only architecture could also include the simplified segmentation model of 64bit for segmentation support for 32bit applications, matching what modern operating systems already use, as well as removing ring 1 and 2 (which are unused by modern software) and obsolete segmentation features like gates and removing 16-bit addressing support.
This would also eliminate support for ring 3 I/O port accesses, string port I/O, which supported an obsolete CPU-driven I/O model, and limit local interrupt controller (APIC) use to X2APIC while removing legacy 8259 support.
While running a legacy 64bit operating system on top of a 64bit mode-only architecture CPU is not an explicit goal of this effort, this could be achieved with the VMX virtualization hardware to emulate the features required to boot legacy operating systems.
From 16bits to 64bits – a timeline
1978 – Intel 8086 released with 16bit real mode.
1982 – Intel 80286 with 16bit protected mode released. Lock prefix #GP if CPL is less privileged than IOPL in the original protected mode architecture.
1985 – Intel 80386 released with 32bit protected mode with paging. Added VM86 mode to support 16-bit real-mode operating system under operating system using paging. Intel 30386 changes the behaviour of lock prefix to ignore IOPL.
1997 – Intel Pentium II introduces SYSENTER extension, codifying flat code and stack segments in an architectural extension.
2004 –64bit Intel Architecture systems released. In 64bit mode, support for VM86 mode dropped.
2005 – Virtualization Technology added to Intel Architecture, enabling running legacy operating systems in virtual machines.
2005 – Windows XP Professional x64 Edition drops support for Windows 16bit binaries.
2008 – A20 gate removed, which dropped compatibility to early 8086 operating system outside emulation.
2012 – 64bit UEFI firmware interfaces widely deployed.
2020 – Intel firmware dropped support for running 16bit/32bit or non-UEFI operating systems natively.
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