The first time I came across the 'turbo frequency' BIOS setting was when I tried out the first of the new ABIT boards. You could enable 'turbo frequency' in the CPU SoftMenu part of the BIOS setup.
The official explaination from the manual is
This item will only be displayed if your CPU external clock supports Turbo mode.
The Turbo mode allows you to speed up the external clock by approximately 2.5%. This feature is used to verify the design flexibility. It is a very important tool for test units to verify CPU stability. Do not use this feature.
Well, 'Do not use this feature' says everything to me. If this isn't hippocritic (spelling?), then I don't know what is. Now since I have been a bad child, always doing what I've been told NOT to do, I've tried this feature several times. In fact, I wouldn't run any CPU for 66 MHz external clock (or bus speed) without this feature. It tweaks only a small bit of performance out of the chip, but it does it reliably and without causing any trouble. The performance increase is definitely measureable and this is no surprise, since it increases the bus as well as the CPU speed, and you hopefully know now, how important the bus speed is.
Although the manual speaks of a constant increase of 2.5% in bus speed, I found out in my measurings, that it more is increasing the CPU clock by a constant 5 MHz. A P133 runs at 138 MHz, a P166 runs at 171 MHz and a P200 runs at 205 MHz. The bus speed always is around 68 MHz.
Now in the SoftMenu, you can't enable 'turbo frequency' for anything else than 66 MHz external clock or bus speed, which seems a little bit unfair to the ones, who run a P75, P90, P120 or P150 and don't want to overclock their CPU to 66 MHz bus speed. The funny thing is, that the PLL (Syntheziser) chip certainly IS able to increase 60 MHz bus speed to 61.5 MHz as well and this is easily seen in the ABIT PR5 board, which does not use the SoftMenu, but still has good old dip switches for changing the CPU parameters. Here you can enable the 'turbo frequency' for 50, 60 and 66 MHz bus speed, which is a blessing for all the above CPU owners.
Are only owners of ABIT boards able to benefit from the 'turbo frequency' feature?
No! In fact it's nothing but a PLL chip thing! You'll need to have either the PLL52C59-14 or the PLL52C61-01 or -21 on your board. If you fiddle around with the different jumper or dip switch settings, you'll find out, which setting you'll require for the 'turbo frequency' mode.
If you want to do it the scientific way:
The turbo frequency jumper is the one jumper of the three, which is OFF or 1-2 at the 50 MHz setting. This jumper has to be ON or 2-3 to enable the turbo frequency for 50, 60 or 66 MHz. This jumper also switches between 55 and 75 MHz in case you have both of the other two jumpers/dip switches at OFF or 1-2. OFF OFF OFF or 1-2 1-2 1-2 for 55 MHz, OFF OFF ON or 1-2 1-2 2-3 for 75 MHz. Please note, that the order and number of the jumpers/dip switches can vary on each board. Hence first find out from the 50 MHz setting, which one the 'turbo jumper/dip switch' is!!! It's the one that's at 50 MHz setting different to the other two.
- PLL52C61-01 or -21
This PLL chip doesn't offer 'turbo frequency' for 50 MHz, but it offers 83 MHz bus speed instead. This however depends on the circutitry of the other pins of that chip. Not all boards support the 'turbo frequency' and not all boards support 83 MHz. Please be aware of that!
- 60 MHz turbo frequency setting:
It's exactly the opposite setting to the normal 60 MHz setting. So if you've e.g. got ON ON OFF or 2-3, 2-3, 1-2, then you have to change to OFF OFF ON or 1-2, 1-2, 2-3 for 60 MHz turbo frequency.
- 66 MHz turbo frequency setting:
All jumpers/dip switches to the setting of the one only jumper, which is different at normal 66 MHz setting. So if you've got OFF ON ON or 1-2, 2-3, 2-3, change it to OFF OFF OFF or 1-2, 1-2, 1-2. This setting e.g. is valid for the Asus P/I-P55T2P4 rev. 3, which normally would have 2-3, 1-2, 2-3 and so needs 1-2, 1-2, 1-2.
- 60 MHz turbo frequency setting:
Please be careful when playing around with the bus speed jumpers!! I am not to be held liable for any damage you may cause to your system!