Content: Information from my own K6 overclocking experiments with Quake benchmarks. FIC VA503+ motherboard undocumented settings.

Allen's Adventures in Overclocking

Notice: This information is provided as-is without warranty of any kind. I am not responsible for any damage to data or to hardware caused by its use. Overclocking is risky and should only be done by experts. Also, I'm not paid or rewarded in any way by any hardware manufacturer.

A Description of My System

FIC VA503+ motherboard with 1 meg pipelined burst cache
AMD K6-300 (It's NOT a K6-2 "3DNow!" chip, and it's external bus is supposed to run at 66MHz.)
The CPU has a 3/4 inch tall heat sink with fan, and I used heat sink compound on it.
I also have a case fan which blows cool air into the bottom of the case near the CPU.
64 meg PC100 DIMM
2m PCI Matrox Mystique video (Strangely, this card won't run on a 25MHz PCI bus.)

Details of My Tests

Quake test: DOS Quake v1.08 (NOT 3D accelerated GLQuake) under Windows 95 command prompt (DOS 7), sound on, timedemo demo2, both status bars on, 320x200.

PSP test: Paint Shop Pro image emboss operation on huge image under Windows 95.

Overclocking Results

This motherboard allows you to set the speed of the CPU external bus separately from the main memory bus. The numbers below are in this format: memory bus/CPU bus/CPU speed.

Quake PSP
66/66/300 57.7 fps 7.9 sec
66/100/300 62.9 fps 7.6 sec
100/100/300 65.3 fps 7.3 sec
66/83/375 67.2 fps Not attempted
83/83/375 71 fps Not attempted
100/100/350 72.5 fps 6.4 sec

The AMD K6-266 and faster CPUs are good overclockers because they use a new fabrication technique. The K6-233 is not as good an overclocking bet, although some can run at 250 or even 262.5.

My K6-300 has been running almost continuously at 300MHz with its external bus overclocked to 100MHz since early July 1998 with no problems. At 350MHz the heat sink gets only slightly warm to the touch and I've played Quake for hours at a time with no crashes or weird behavior.

I completed the timedemo test with Quake at 375 MHz, but it locked up or had a page fault after about five minutes. Clearly the CPU was heating up too much.

I've connected my turbo button to one of the multiplier jumpers so I can switch between 300 and 350 MHz. The motherboard doesn't change the setting on the fly. I have to punch the button and then restart the computer.

The Value of PC100 Memory

These results show that this motherboard's ability to run the CPU bus (and its one meg L2 cache, I assume) faster than main memory gets you 2/3 of the performance boost of having PC100 memory. If you already have lots of 66MHz memory, this motherboard lets you keep using it with very little reduction in performance. But, if you need to buy new memory, I'd recommend getting PC100 memory since it isn't much more expensive and will probably save you from having to buy new memory for your next motherboard upgrade.

Undocumented jumper settings of the FIC VA503+

The VA503+ manual tells you how to set the jumpers, but it doesn't tell you exactly what the jumpers do. Those of us who want to push our systems to their limits need this information. Here's what they left out.

The motherboard has 4 speeds you can set:

  • CPU external bus speed
  • CPU internal speed
  • PCI/AGP bus speeds (The AGP bus is always twice the PCI bus.)
  • Memory bus speed
  • On all the 3 pin jumpers, pin 1 is the pin closest to the thick white line on the motherboard. In my tables, "1-2" and "2-3" mean the jumper is on those two pins.

    The CLK1, CLK2, and CLK3 jumpers set the CPU external bus speed from 60MHz to 124MHz. Many of these settings are undocumented.

    CPU External Bus Speed
    CLK1 CLK2 CLK3 MHz
    2-3 1-2 1-2 60
    1-2 1-2 1-2 66
    2-3 2-3 2-3 68.5
    1-2 2-3 1-2 75
    2-3 2-3 1-2 83
    2-3 1-2 2-3 95
    1-2 1-2 2-3 100
    1-2 2-3 2-3 112
    2-3 1-2 1-2 124

    The FREQ1, FREQ2, and FREQ3 jumpers set what the CPU external bus speed is multiplied by to get the CPU internal speed. All of these are official, documented settings.

    CPU External to Internal Multiplier
    FREQ1 FREQ2 FREQ3 Multiply by
    2-3 1-2 1-2 2
    2-3 2-3 1-2 2.5
    1-2 2-3 1-2 3
    1-2 1-2 1-2 3.5
    2-3 1-2 2-3 4
    2-3 2-3 2-3 4.5
    1-2 2-3 2-3 5
    1-2 1-2 2-3 5.5

    The SDRAM1 and CLK4 jumpers set whether the memory bus speed is set to the AGP bus speed or the CPU external speed. This allows you, for example, to run your CPU bus at 100MHz and your memory bus at 66MHz.

    1-2 2-3 Memory=AGP bus
    2-3 1-2 Memory=CPU external bus

    The SDRAM2 and SDRAM3 jumpers select what the CPU external bus is divided by to generate the PCI bus. The PCI bus is supposed to be 33.3MHz, but can be run faster if your PCI cards can handle it. For example, if you're running the CPU external bus at 100MHz you would normally set the PCI bus to divide by 3. If the CPU external bus is at 75MHz, you can set the PCI bus to 75/2.5=30 or 75/2=37.5. The AGP bus is always twice the PCI bus. The divide by 2.5 setting is undocumented and may not work on all VA503+ motherboards.

    SDRAM2 SDRAM3 Divide by:
    2-3 2-3 2
    1-2 1-2 2.5
    1-2 2-3 3
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