VOGONS


First post, by Daastaja

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I am thinking of underclocking my 486 DX4 currently running at 100Mhz. The purpose would be to be able to play some speed sensitive games without using Moslo and such programs.

Would it be safe to set the jumpers (see picture 1 for the layout info and picture 2 for my current jumper layout) for eg. to the 486 SX settings? Disabling the caches in BIOS brings the speed down too much and the slow mode (turbo off) does the same thing. And all the slowdown software seems to lag more or less (I have really tried most of them).

Btw. I had an old account and had to recreate it for a new e-mail account as it probably had been suspended due to lack of logins for a while... Hence the first post 😀

Reply 1 of 11, by Tiido

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DX4 has its multiplier controlled by a jumper, it can be 2x or 3x. I'm not sure which one it would be from those jumpers shown. Bus speed jumpers will let you get slower speeds, there's sometimes speeds available slower than 25MHz. I have one board that has 4, 8 and 16MHz too in addition to 25, 33, 40 and 50MHz.

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Reply 2 of 11, by Daastaja

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Yeah, kind of thought so. I am quessing those jumpers would control the bus speeds, but the instructions on the mobo are just dumbed up versions of those...

Reply 3 of 11, by Daastaja

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After some research it's this mobo: PC Сhips 486 M912 v1.4

Reply 4 of 11, by mkarcher

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According to https://www.elhvb.com/mobokive/Archive/Oldman … ips/jumpers.htm, linked from PC Chips M912 BIOS update for Am5x86 and Cyrix 5x86 , the only jumper that differs between AMD 5x86-133 and AMD WriteBack Enhanced DX4 is JP31, which is open for DX4 and 2-3 for 5x86-133. This is obviously the pin you use to choose between x3 clock for the DX4 or x4 clock for the 5x86. This is the same pin that chooses between x2 and x3 clock on the Intel DX4, so jumpering JP31 to 2-3 should turn your DX4 processor into a DX2 processor.

Do not blindly jumper the processor like a 486DX/486DX2. You need to keep the voltage jumpers at 3.3V, or you might damage the processor.

That link also shows you how to jumper JP6/JP7/JP8 for 25MHz bus clock, which turns your DX4-100 into a DX4-75 (or DX2-50, if you set JP31). The clock chip on your board is a UMC9515-01, which is a clone of the Macronix MX8315. From the MX8315 datasheet, you can deduce that you get 20 MHz bus clock (DX2-40/DX4-60) if you short all of JP6, JP7 and JP8. There is no slower setting.

Reply 5 of 11, by Daastaja

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Thanks, really appreciated!

Reply 6 of 11, by Daastaja

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JP31 both 2 positions gave me 66Mhz. I also tried shorting only JP8 instead of initial 6-8 and got no boot. Well, I guess I am stuck with using slowdown software. Thanks anyway!

Reply 7 of 11, by mkarcher

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Daastaja wrote on 2020-12-25, 21:36:

JP31 both 2 positions gave me 66Mhz. I also tried shorting only JP8 instead of initial 6-8 and got no boot. Well, I guess I am stuck with using slowdown software. Thanks anyway!

Oops, I was wrong. 20MHz is JP6-8 all open, not all closed. You might give that one a try.

Reply 8 of 11, by Daastaja

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6-8 open gave me only black screen.

It seems the combos are a bit different on that mobo. https://www.elhvb.com/webhq/models/486vlb3/m912v14.htm From this link =>

JP6 JP7 & JP8 - CPU Clock Selection Jumper TOP
==================================================
CPU Clock CPU Frequency JP6 JP7 JP8 ----------------------------------------------------------------------------------------------------
25MHz DX2-50MHz OPEN OPEN SHORT
33MHz DX2-66MHz SHORT SHORT SHORT
DX4-100MHz
40MHz DX-40MHz OPEN SHORT SHORT
50MHz DX-50MHz SHORT OPEN OPEN
==================================================

Reply 9 of 11, by mkarcher

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Daastaja wrote on 2020-12-26, 10:07:
6-8 open gave me only black screen. […]
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6-8 open gave me only black screen.

It seems the combos are a bit different on that mobo. https://www.elhvb.com/webhq/models/486vlb3/m912v14.htm From this link =>

JP6 JP7 & JP8 - CPU Clock Selection Jumper
===================================================
CPU Clock CPU Frequency JP6 JP7 JP8
---------------------------------------------------
25MHz DX2-50MHz OPEN OPEN SHORT
33MHz DX2-66MHz SHORT SHORT SHORT
DX4-100MHz
40MHz DX-40MHz OPEN SHORT SHORT
50MHz DX-50MHz SHORT OPEN OPEN
===================================================

If you just look at "CPU Clock", "JP6", "JP7" and "JP8", the table I linked (from the 1.7 manual) and the table you quoted (from the 1.4 manual) are identical. The column "CPU frequency" is just for your information to tell you what CPUs run at the given external clock. As the version 1.7 is newer, the table lists a couple of CPUs that did not exist when version 1.4 was produced. I can't explain why the system won't boot at lower clock settings than 33MHz, but maybe the BIOS tries to auto-detect the CPU and bus clock and fails at unconventional settings (DX4-75 or DX4-60). At least the DX2-50 combination is supposed to work, because that's an officially supported processor. This means JP6 open, JP7 open, JP8 short, JP31 2-3.

To further slow down your system without resorting to completely disable the cache, you can try to increase the L2 cache wait states (read wait state: 3-2-2-2, write wait state 1).

Reply 10 of 11, by Daastaja

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Thanks man, great tips! will try that later on.

I got a kind of scared earlier as I was trying the jp 6-8 combos and got it booting at 120 mhz., hehe. =>rushed to power button.

Reply 11 of 11, by Daastaja

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A quick update: JP6 open, JP7 open, JP8 short, JP31 2-3 black screened. It might be as you said there that there is some cpu auto detection issue. I will try the wait states still.