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The Official "Overclocking" FAQ
Comp.sys.ibm.pc.hardware.chips "Overclocking" FAQ version 0.5.2
This FAQ is maintained by HB Papaleonardos, email@example.com. Any suggestions, comments, questions, or additions are welcome.
1.1. What is "overclocking?"
One can also "overclock" the computer's bus. See question 2.4 ("Any other settings to change?") for details.
1.2. What chips can be "overclocked?"
2.1. How do I "overclock?"
For #1, your oscillator module (clock crystal) will be a little silver box soldered to the motherboard, possibly strapped to the board with a little plastic fastener.
- If you have a 386, the oscmod will be labeled with twice your clock speed. Example: if you have a 25 MHz machine, the oscmod will be 50 MHz.
- If you have a regular 486 (not clock doubled or tripled) processor with an older motherboard, probably the oscmod will be labeled with the speed of your processor. Example: 33 MHz machine has a 33 MHz oscmod.
- If you have an older but better 486 motherboard, your oscmod will be twice the speed of the processor. The motherboard divides the frequency by two to get better accuracy. Example: if you have a DX-33, then your oscmod would be 66 MHz.
- If you have a DX/2 or other clock doubled processor, the oscmod will show half your clock speed. For example, if you have a DX/2-66, then your oscmod will be 33 MHz.
Just cut the fastener, desolder the oscmod if necessary, and replace it with a new oscmod. You will probably want the 4-pin oscillators (they are often described as fitting in a 14 pin DIP socket). See question 2.2 for oscmod sources. It is recommended that you save your old oscmod in case you have to go back to that speed.
For #2, just change the system speed jumpers. See your motherboard manual, call your computer's technical support number, or see question 2.3 as a last resort to find out what they are.
2.2. Where can I get a new oscmod?
DigiKey (+1 800 344 4539)
Jameco (+1 800 237 6948)
Mouser Electronics (+1 800 346 6873)
You will be looking for full-size TTL clock oscillators. Telll them that, or oscmods, but NOT crystals, which are 2 pins only and won't work
2.3. What are typical jumper settings?
2.4. Any other settings to change?
Bus "overclocking" may not be as dangerous. You go into your BIOS and change your CLK/x to a higher ratio (a lower x). If you have problems, either try to eliminate slow devices, or raise the x value. It only takes one device which can't handle the higher speed to make your system unstable. Incidentally, ISA standards exist for bus speeds as high as 16 MHz, [if you know of a published timing specification for "16 MHz" ISA, please mail me, firstname.lastname@example.org (Cameron)] even though the most common is only 8 MHz. Again, I recommend that you see the "BIOS Survival Guide."
2.5. What are speed changes are typically made?
There are no intentions to add "real-life stories" of "overclocking" to this FAQ; such stories may be appropriate for another document.
2.6. What are typical performance improvements?
The non-fatal one is due to timing. Processors are designed and tested so that their internal parts will all be ready about the same time, according to specifications published by their manufacturer. As the heat of the processor increases above specifications, the internal paths slow down. Some paths slow down more than others, and eventually there becomes a significant difference between when something is expected to happen and when it actually does happen. At this point you may get false information (such as 0+0=1), system lock-ups, or spontaneous resets. This behavior is usually a signal for you to decrease processor speed or temperature (see next section).
One fatal possibility is called electromigration. From Cameron
They tend to grow little whiskers at any sharp corner or irregularity along a trace. Whiskers at different potentials tend to grow towards each other, much as stalactites grow towards stalagmites in a limestone cave, because the sharp point accentuates the potential gradient. In a cave, limestone columns eventually form from floor to ceiling. On a chip, you get a short circuit.
(Heat is only a secondary factor in electromigration. The primary factor is current density in the presence of an electric field. In most digital ICs, the internal clock signal is distributed by a conductor, usually aluminum, which is sized carefully for its load. If the clock switches more often than the designer sized it for, then the clock "trunk" is overloaded and subject to premature failure due to electromigration.)
The other fatal possibility is simply burning out the bond wires that run from the pins on the outside of the package to the silicon die of the processor.
BTW, these problems with heat have nothing to do with the die
melting. According to Nick Paizis, most often silicon remains
functional well above 125 degrees C. The key is to not exceed the
maximum recommended operating temperature.
3.2. What are some other problems?
The other device-based problem is with your motherboard. Every cheap little component on the board must function reliably at the higher speed or you will likely experience reliability problems. The only reasonable solution to this problem is to get a faster motherboard.
4.1. What can I do to reduce CPU temperature?
They are rather expensive. You still need a fan, and probably a heat sink, when you use one.
4.2. What else about fans and heat sinks?
If you are concerned about heat transfer efficiency in your combo
heatsink and fan, perform this simple experiment (Richard Knipe):
If the heatsink never gets hot without the fan running, then either your chip doesn't generate much heat or the thermal contact between the heatsink and chip is poor--and you better find out which, else... bad luck! If the heatsink gets really hot without the fan running, the thermal contact is good.
There is some room for concern regarding the CPU fans (the ones that clip directly onto the chip). Some suspect that the vibrations from the fan may damage the processor, or slowly cause a bad connection between the chip and its socket (Natsuki Ishizuka). In this case it may be better to mount the fan somewhere else in the case. But be careful!
4.3. Where can I get all this stuff?
Their product is marketted under ARX CPU Cooler. The part I'd recommend is ACC4846-23/231 and ACC4846-30/301 (with slightly better cooling power) for 486 and ACC5856-30TC and ACC5856-40TC (again with slightly better cooling power). (WH)
Alpha & Omega Computer
Data Products DPS
PC Power & Cooling
[Additions to this section are always welcome!]
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