&os;/i386 runs on a wide variety of IBM PC compatible machines. Due to the wide range of hardware available for this architecture, it is impossible to exhaustively list all combinations of equipment supported by &os;. Nevertheless, some general guidelines are presented here. Almost all i386-compatible processors are supported. All Intel processors beginning with the 80386 are supported, including the 80386, 80486, Pentium, Pentium Pro, Pentium II, Pentium III, Pentium 4, and variants thereof, such as the Xeon and Celeron processors. (However, &os; 5.2-RELEASE and later no longer support the 80386SX processor.) All i386-compatible AMD processors are also supported, including the Am486, Am5x86, K5, K6 (and variants), Athlon (including Athlon-MP, Athlon-XP, Athlon-4, and Athlon Thunderbird), and Duron processors. The AMD Élan SC520 embedded processor is supported. The Transmeta Crusoe is recognized and supported, as are i386-compatible processors from Cyrix and NexGen. There is a wide variety of motherboards available for this architecture. Motherboards using the ISA, VLB, EISA, AGP, and PCI expansion busses are well-supported. There is some limited support for the MCA (MicroChannel) expansion bus used in the IBM PS/2 line of PCs. Symmetric multi-processor (SMP) systems are generally supported by &os;, although in some cases, BIOS or motherboard bugs may generate some problems. Perusal of the archives of the &a.smp; may yield some clues. &os; will take advantage of HyperThreading (HTT) support on Intel CPUs that support this feature. A kernel with the options SMP feature enabled will automatically detect the additional logical processors. The default &os; scheduler treats the logical processors the same as additional physical processors; in other words, no attempt is made to optimize scheduling decisions given the shared resources between logical processors within the same CPU. Because this naive scheduling can result in suboptimal performance, the logical CPUs are halted by default at startup. They can be enabled with the machdep.hlt_logical_cpus sysctl variable. It is also possible to halt any CPU in the idle loop with the machdep.hlt_cpus sysctl variable. The &man.smp.4; manual page has more details. &os; will take advantage of Physical Address Extensions (PAE) support on CPUs that support this feature. A kernel with the PAE feature enabled will detect memory above 4 gigabytes and allow it to be used by the system. This feature places constraints on the device drivers and other features of &os; which may be used; consult the &man.pae.4; manpage for more details. &os; will generally run on i386-based laptops, albeit with varying levels of support for certain hardware features such as sound, graphics, power management, and PCCARD expansion slots. These features tend to vary in idiosyncratic ways between machines, and frequently require special-case support in &os; to work around hardware bugs or other oddities. When in doubt, a search of the archives of the &a.mobile; may be useful. Most modern laptops (as well as many desktops) use the Advanced Configuration and Power Management (ACPI) standard. &os; supports ACPI via the ACPI Component Architecture reference implementation from Intel, as described in the &man.acpi.4; manual page. The use of ACPI causes instabilities on some machines and it may be necessary to disable the ACPI driver, which is normally loaded via a kernel module. This may be accomplished by adding the following line to /boot/device.hints: hint.acpi.0.disabled="1" Users debugging ACPI-related problems may find it useful to disable portions of the ACPI functionality. The &man.acpi.4; manual page has more information on how to do this via loader tunables. ACPI depends on a Differentiated System Descriptor Table (DSDT) provided by each machine's BIOS. Some machines have bad or incomplete DSDTs, which prevents ACPI from functioning correctly. Replacement DSDTs for some machines can be found at the DSDT section of the ACPI4Linux project Web site. &os; can use these DSDTs to override the DSDT provided by the BIOS; see the &man.acpi.4; manual page for more information.