ramashiva

In my previous post, "amd k6-III overclocking and performance issues", I stated that I could set the memory-bus speed to 124 mhz on my FIC VA-503A motherboard. That is true, but I neglected to mention that a couple of tweaks were necessary to achieve stable operation at 124 mhz.

First, I had to raise the CPU voltage from 2.2 volts to 2.6 volts to get the system to POST. Strange, you say? I thought so, too. If you recall, I'm running my k6-III at 504 mhz (4.5x112). That's a clock multiplier of 4.5 and a FSB speed of 112 mhz. The CPU is very stable at this speed at 2.2 volts, which is its rated voltage. But the FSB speed of 112 mhz is a BIOS setting. When the system begins the POST operation, apparently the dipswitch settings for clock multiplier and memory-bus speed are the determining factors. So I have to supply the CPU with enough voltage to run at 4.5 x 124 mhz = 558 mhz. That voltage requirement is 2.6 volts.

Second, I had to disable spread spectrum modulation. This setting is located in the BIOS under the Chipset Features Setup heading. I am pretty sure disabling spread spectrum modulation will increase memory stability on any motherboard at memory-bus speeds above 100 mhz.

I think it is safe to generalize that, on any MVP3-based motherboard, if you set the FSB speed in the BIOS below the memory-bus speed set by jumpers or dipswitches, then you must supply the CPU with sufficient voltage to POST at the product of the clock multiplier and the memory-bus speed.

Running my memory bus at 124 mhz results in slight, but measurable increases in CPU performance and memory bandwidth. However, I have chosen to run my memory bus at 112 mhz so I can keep the voltage set at 2.2 volts. I'm pretty sure cranking up the voltage on any CPU shortens the trouble-free life of that CPU, even when well-cooled. I'm willing to sacrifice CPU life for a better overclock, but not for marginally better SANDRA scores.

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William Coleman