12 Support for Multiple Architectures

FreeBSD is a highly portable operating system intended to function on many different types of hardware architectures. Maintaining clean separation of Machine Dependent (MD) and Machine Independent (MI) code, as well as minimizing MD code, is an important part of our strategy to remain agile with regards to current hardware trends. Each new hardware architecture supported by FreeBSD adds substantially to the cost of code maintenance, toolchain support, and release engineering. It also dramatically increases the cost of effective testing of kernel changes. As such, there is strong motivation to differentiate between classes of support for various architectures while remaining strong in a few key architectures that are seen as the FreeBSD "target audience".

12.1 Statement of General Intent

The FreeBSD Project targets "production quality commercial off-the-shelf (COTS) workstation, server, and high-end embedded systems". By retaining a focus on a narrow set of architectures of interest in these environments, the FreeBSD Project is able to maintain high levels of quality, stability, and performance, as well as minimize the load on various support teams on the project, such as the ports team, documentation team, security officer, and release engineering teams. Diversity in hardware support broadens the options for FreeBSD consumers by offering new features and usage opportunities (such as support for 64-bit CPUs, use in embedded environments, etc.), but these benefits must always be carefully considered in terms of the real-world maintenance cost associated with additional platform support.

The FreeBSD Project differentiates platform targets into four tiers. Each tier includes a specification of the requirements for an architecture to be in that tier, as well as specifying the obligations of developers with regards to the platform. In addition, a policy is defined regarding the circumstances required to change the tier of an architecture.

12.2 Tier 1: Fully Supported Architectures

Tier 1 platforms are fully supported by the security officer, release engineering, and toolchain maintenance staff. New features added to the operating system must be fully functional across all Tier 1 architectures for every release (features which are inherently architecture-specific, such as support for hardware device drivers, may be exempt from this requirement). In general, all Tier 1 platforms must have build and tinderbox support either in the FreeBSD.org cluster, or easily available for all developers.

Tier 1 architectures are expected to be Production Quality with respects to all aspects of the FreeBSD operating system, including installation and development environments.

Current Tier 1 platforms are i386, Sparc64, AMD64, and PC98.

12.3 Tier 2: Developmental Architectures

Tier 2 platforms are not supported by the security officer and release engineering teams. At the discretion of the toolchain maintainer, they may be supported in the toolchain. New features added to FreeBSD should be feasible to implement on these platforms, but an implementation is not required before the feature may be added to the FreeBSD source tree. The implementation of a Tier 2 architecture may be committed to the main FreeBSD tree as long as it does not interfere with production work on Tier 1 platforms, or substantially with other Tier 2 platforms. Before a Tier 2 platform can be added to the FreeBSD base source tree, the platform must be able to boot to at least single-user mode on real world commodity hardware. Some exceptions to these rules may be made for new hardware that is under development by hardware vendors, but not yet available to the project.

Tier 2 architectures are usually systems targeted at Tier 1 support, but that are still under development. Architectures reaching end of life may also be moved from Tier 1 status to Tier 2 status as the availability of resources to continue to maintain the system in a Production Quality state diminishes.

Current Tier 2 platforms are PowerPC and ia64.

12.4 Tier 3: Experimental Architectures

Tier 3 platforms are not supported by the security officer and release engineering teams. At the discretion of the toolchain maintainer, they may be supported in the toolchain. Tier 3 platforms are architectures for which hardware is not or will not be available to the project in the foreseeable future, for which there are two or fewer active developers, that can not boot to at least single-user mode on real hardware (or a simulator for new hardware platforms), or which are considered legacy systems unlikely to see broad future use. Tier 3 systems will not be committed to the base source tree, although support for Tier 3 systems may be worked on in the FreeBSD Perforce Repository, providing source control and easier change integration from the main FreeBSD tree.

Current Tier 3 platforms are S/390®.

12.5 Tier 4: Unsupported Architectures

Tier 4 systems are not supported in any form by the project.

All systems not otherwise classified into a support tier are Tier 4 systems.

12.6 Policy on Changing the Tier of an Architecture

Systems may only be moved from one tier to another by approval of the FreeBSD Core Team, which shall make that decision in collaboration with the Security Officer, Release Engineering, and toolchain maintenance teams.

This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.

For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.