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Jamicon KM-S4-1 / Rectron RT-4S3 Speed PCI-slot going down after overclock

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First post, by egbertjan

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I have a motherboard KM-S4-1 with a AM5x86-P75-S CPU. The most recent BIOS (4.2) is installed. Normally this CPU is operated at a clockspeed of 133 MHz. I have overclocked the CPU by rising the busspeed from 33 to 40 MHz, so that the CPU works on 160 MHz. After I raised the CPU-voltage to 4 V the system works stable as far as I can see. An unexpected effect after overclocking is that the speed of the PCI slot with the videocard (a S3 Virge DX) is going down. I saw that by using the program Speedsys before and after the overclock. The photo's below show that the speed went down from 12300 to 11120 kb/s.

Before overclock:

133mhz.jpg
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133mhz.jpg
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After overclock:

160mhz.jpg
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160mhz.jpg
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1.94 MiB
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278 views
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Fair use/fair dealing exception

Playing games that ask a lot of video-capacity (Duke Nukem 3d or Doom) performed less good after the overclock, "light" games performed much better.

Sometime ago I used the motherboard PVI-486sp3 that has the same motherboard chip. There the videocard performed a lot better with a speed of 22243 kb/s. I used the same videocard as above.

Does someone know why the KM-S4-1 is so underperforming?

Reply 1 of 2, by mkarcher

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Probably, when you changed the FSB clock to 40 MHz from 33 MHz, you also enabled the PCI clock divider. This means you down-clocked the PCI bus from 33 MHz to 20 MHz. On SiS 496/497 boards, the PCI clock divider is configured using a jumper. The bus clock configuration shown in mainboard manuals usually include the position for the "PCI-divide-by-2" jumper. Try setting JP12 as indicated for 33MHz, but keeping JP11 as indicated for 40MHz.

Reply 2 of 2, by egbertjan

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Thank you mkarcher. As you adviced I changed the setting of JP12 to 33 MHZ. The result was a very little increase of the speed of the videocard. The same videocard on another motherboard with the same chipset shows a much higher speed: 16600 in stead of 14805 KB/s. Are there more options to increase the speed of the videocard on the KM-S4-1 or should I be happy with what I've got?

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