D.3. Core/Processing

D.3.1. Motherboards, busses, and chipsets

D.3.1.4. PCI

Contributed by David O'Brien from postings by Rodney Grimes . 25 April 1995.

Continuing updates by Jordan K. Hubbard . Last update on 26 August 1996.

Of the Intel PCI chip sets, the following list describes various types of known-brokenness and the degree of breakage, listed from worst to best.


Cache coherency problems, especially if there are ISA bus masters behind the ISA to PCI bridge chip. Hardware flaw, only known work around is to turn the cache off.

Saturn-I (ie, 82424ZX at rev 0, 1 or 2):

Write back cache coherency problems. Hardware flaw, only known work around is to set the external cache to write-through mode. Upgrade to Saturn-II.

Saturn-II (ie, 82424ZX at rev 3 or 4):

Works fine, but many MB manufactures leave out the external dirty bit SRAM needed for write back operation. You can work around this either by running it in write through mode, or get the dirty bit SRAM installed (I have these for the ASUS PCI/I-486SP3G rev 1.6 and later boards).


Can not run more than 2 bus master devices. Admitted Intel design flaw. Workarounds include do not run more than 2 bus masters, special hardware design to replace the PCI bus arbiter (appears on Intel Altair board and several other Intel server group MB's). And of course Intel's official answer, move to the Triton chip set, we "fixed it there".

Triton (ie, 430FX):

No known cache coherency or bus master problems, chip set does not implement parity checking. Workaround for parity issue. Use Triton-II based motherboards if you have the choice.

Triton-II (ie, 430HX):

All reports on motherboards using this chipset have been favorable so far. No known problems.


Early versions of this chipset suffered from a PCI write-posting bug which can cause noticeable performance degradation in applications where large amounts of PCI bus traffic is involved. B0 stepping or later revisions of the chipset fixed this problem.


This Pentium Pro support chipset seems to work well, and does not suffer from any of the early Orion chipset problems. It also supports a wider variety of memory, including ECC and parity. The only known problem with it is that the Matrox Meteor frame grabber card doesn't like it.

D.3.2. CPUs/FPUs

Contributed by Satoshi Asami . 26 December 1997.

D.3.2.1. P6 class (Pentium Pro/Pentium II)

Both the Pentium Pro and Pentium II work fine with FreeBSD. In fact, our main FTP site ftp.FreeBSD.org (also known as "ftp.cdrom.com", world's largest ftp site) runs FreeBSD on a Pentium Pro. Configurations details are available for interested parties.

D.3.2.2. Pentium class

The Intel Pentium (P54C), Pentium MMX (P55C), AMD K6 and Cyrix/IBM 6x86MX processors are all reported to work with FreeBSD. I will not go into details of which processor is faster than what, there are millions of web sites on the Internet that tells you one way or another. :)

Note: Various CPUs have different voltage/cooling requirements. Make sure your motherboard can supply the exact voltage needed by the CPU. For instance, many recent MMX chips require split voltage (e.g., 2.9V core, 3.3V I/O). Also, some AMD and Cyrix/IBM chips run hotter than Intel chips. In that case, make sure you have good heatsink/fans (you can get the list of certified parts from their web pages).

D. Clock speeds

Contributed by Rodney Grimes . 1 October 1996.

Updated by Satoshi Asami . 27 December 1997.

Pentium class machines use different clock speeds for the various parts of the system. These being the speed of the CPU, external memory bus, and the PCI bus. It is not always true that a "faster" processor will make a system faster than a "slower" one, due to the various clock speeds used. Below is a table showing the differences:

Rated CPU MHzExternal Clock and Memory Bus MHzExternal to Internal Clock MultiplierPCI Bus Clock MHz
150602.530 (Intel, AMD)
15075237.5 (Cyrix/IBM 6x86MX)

Note: 66MHz may actually be 66.667MHz, but don't assume so.

The Pentium 100 can be run at either 50MHz external clock with a multiplier of 2 or at 66MHz and a multiplier of 1.5.

As can be seen the best parts to be using are the 100, 133, 166, 200 and 233, with the exception that at a multiplier of 3 or more the CPU starves for memory.

D. The AMD K6 Bug

In 1997, there have been reports of the AMD K6 seg faulting during heavy compilation. That problem has been fixed in 3Q '97. According to reports, K6 chips with date mark "9733" or larger (i.e., manufactured in the 33rd week of '97 or later) do not have this bug.

D.3.2.5. 286 class

Sorry, FreeBSD does not run on 80286 machines. It is nearly impossible to run today's large full-featured unices on such hardware.

D.3.3. * Memory

The minimum amount of memory you must have to install FreeBSD is 5 MB. Once your system is up and running you can build a custom kernel that will use less memory. If you use the boot4.flp you can get away with having only 4 MB.

D.3.4. * BIOS