Enabling high multipliers on Athlon XP CPUs


There's a lot of information out on the internet about using the multiplier range 5.0-12.5 on CPUs with the 5th multiplier bit set high and a motherboard with only 4 bits supported. But I could not find very much on using the high multipliers (13.0-24.0) when the 5th multiplier bit defaults to low (again, on a board with only 4 bits supported). This is important mainly for using newer CPUs in older boards. This is also important for when you want to use recent XP CPUs (particularily the popular Barton 2500's) in SMP boards. Most new CPUs run on a 166MHz FSB, and most SMP boards only support FSBs up to 150 MHz and 4-bit multipliers. To use a faster CPU in this situation requires setting the 5th multiplier bit high.


The most popular approach seems to be cutting the 5th and 3rd L3 bridges. This is a slightly scary proposition for someone not used to doing this sort of thing, and requires a fair amount of care and patience with the new, laminated, packaging. The other option, which seems to be very rarely mentioned, is doing pinmods with either conductive paint or little bits of wire in the socket. This is the method I used with success (the paint in my case).

And it's easy to undo as well of you make a mistake: just get a wooden barbecue skewer, dab some methylated spirits on the area using a tissue, and scrape gently with the skewer. Scrape with the barbecue skewer almost flat to the surface of the chip (lying between the pins), as then it's almost impossible to damage the chip or pins. Although the skewer is softer than the chip, you still can cause some damage if you poke directly down on the chip. I won't go into details of how to apply the paint, as there are many sites out there that give good instructions on doing it.

The options

In the following pin diagrams, the point of view is from the pin side of the chip (or solder side of the motherboard, if that is what you are modding), and the two "key" pins on the perimiter of the chip are at the bottom. So the chip is aligned like:

The red area is the zoomed-in view shown in the pin diagrams.

The 5-only pinmod

By pulling only the 5th multiplier high, you get access to the full range of multipliers greater than 12.5:
Real multiplier BIOS multiplier

To be able to boot using this mod, you CPU must be able to handle the "high" multiplier equavalent of its default multiplier. This is as follows:
CPU model Default multiplier High multiplier equavalent FSB to run at stock
1500 (1333 MHz)10.02358 MHz
1600 (1400 MHz)10.52458 MHz
1700 (1467 MHz)11.0invalid (won't boot)N/A
2500 (1833 MHz)11.0invalid (won't boot)N/A
3200 (2200 MHz)11.0invalid (won't boot)N/A
1800 (1533 MHz)11.51981 MHz
2600 (Barton, 1917 MHz)11.519101 MHz
1900 (1600 MHz)12.0invalid (won't boot)N/A
2000 (1667 MHz)12.52083 MHz
2600 (TBred, 166MHz FSB, 2083 MHz)12.520104 MHz

Obviously, unless the FSB is significantly reduced, the CPU is unlikely to boot with only the 5th bit pulled high.

The 5-3 pinmod

By pulling the 5th and 3rd multiplier bits high, you get the following options for the multipliers:
Real multiplier BIOS multiplier

Your CPU must be capable of booting at a 13x multiplier for this pinmod to work.

The 5-4 pinmod

By pulling the 5th and 4th multiplier bits high, you get some slightly different multipliers available:
Real multiplier BIOS multiplier

You CPU must be capable of booting at a 15x multiplier for this pinmod to work.

The 5-1 pinmod

The final other option is to pull the 5th and 1st multiplier bits high. This gives you the following multipliers:
Real multiplier BIOS multiplier

You CPU must be capable of booting at a 19x multiplier for this pinmod to work. Because of this, the 13.5-18.0 multipliers are, effectively, useless, so the only useful multipliers are 19, 20, 21, 22, and 24.

Relative usefulness of the pinmods

A graphical view of can be shown by placing a vertical dash at each available CPU speed, at a range of FSB speeds. The lighter the colour of the dash, the more ways there are to get that particular speed. The 6 horozontal bars are, from top to bottom: all multipliers, low only, high only, 5-3 pinmod, 5-4 pinmod, 5-1 pinmod.

For a 100-112 MHz FSB, changable in 1MHz steps:

For a Barton-cored CPU, the CPU typically tops out somewhere between 2000 and 2400 MHz, depending on the CPU, cooling, etc. The 5-3 pinmod clearly offers the best range of speeds in this region.

For a 133-150 MHz FSB, changable in 1MHz steps:

Here, the 5-4 pinmod very nicely covers the required range for a Barton-core CPU, so is probably the best choice. The 5-3 pinmod is only useful if the CPU cannot reach 2000 MHz, as it gives a very good covering of the 1800-2000 MHz range.

One particular FSB set of interest is that obtainable by the K7D-Master. It can only provide FSB speeds of 133, 138, 140, 144, and 150 MHz. Using only these speeds, and zooming in to the 2000-2600MHz range (the typical range for a Barton core CPU) gives the following picture:

Once again, the 5-4 pinmod gives the widest range, unless the CPU tops out close to 2000 MHz, in which case the 5-3 pinmod provides a few more options.