mainly written by Intel's engineers, provides an open-source platform-independent reference implementation of the operating system–related ACPI code. The ACPICA code is used by Linux, Haiku, ArcaOS and FreeBSD, which supplement it with their operating-system specific code. History The first revision of the ACPI specification was released in December 1996, supporting 16, 24 and 32-bit addressing spaces. It was not until August 2000 that ACPI received 64-bit address support as well as support for multiprocessor workstations and servers with revision 2.0. In 1999, then Microsoft CEO Bill Gates stated in an e-mail that Linux would benefit from ACPI without them having to do work and suggested to make it Windows-only. In September 2004, revision 3.0 was released, bringing to the ACPI specification support for SATA interfaces, PCI Express bus, multiprocessor support for more than 256 processors, ambient light sensors and user-presence devices, as well as extending the thermal model beyond the previous processor-centric support. Released in June 2009, revision 4.0 of the ACPI specification added various new features to the design; most notable are the USB 3.0 support, logical processor idling support, and x2APIC support. Revision 5.0 of the ACPI specification was released in December 2011, which added the ARM architecture support. The revision 5.1 was released in July 2014. The latest specification revision is 6.5, which was released in August 2022. Operating systems Microsoft's Windows 98 was the first operating system to implement ACPI, but its implementation was somewhat buggy or incomplete, although some of the problems associated with it were caused by the first-generation ACPI hardware. Other operating systems, including later versions of Windows, macOS, eComStation, ArcaOS, FreeBSD (since FreeBSD 5.0), NetBSD (since NetBSD 1.6), OpenBSD (since OpenBSD 3.8), HP-UX, OpenVMS, Linux, GNU Hurd and PC versions of Solaris, have at least some support for ACPI. Some newer operating systems, like Windows Vista, require the computer to have an ACPI-compliant BIOS, and since Windows 8, the S0ix/Modern Standby state was implemented. Windows operating systems use acpi.sys to access ACPI events. The 2.4 series of the Linux kernel had only minimal support for ACPI, with better support implemented (and enabled by default) from kernel version 2.6.0 onwards. Old ACPI BIOS implementations tend to be quite buggy, and consequently are not supported by later operating systems. For example, Windows 2000, Windows XP, and Windows Server 2003 only use ACPI if the BIOS date is after January 1, 1999. Similarly, Linux kernel