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Overclocking DX4 100MHZ to 120?

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First post, by Charleston

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My mainboard is a V4P895P3 / SMT V: 1.0

I own an AMD 5x86 133mhz but this board does not seem to support it as the max clock is 3x and there is no 4x option as far as I can tell.

https://www.elhvb.com/webhq/models/486vlb3/v4p895v1.htm

Setting the bus speed to 40mhz with the dx4, the computer will boot and start it's normal process, but will still read 100MHZ on the post screen. Once it starts to go through the config and autoexec it hangs and seemingly gives up completely.

Is this an issue with the board? What would be a recommended board for running a 133mhz amd oced to 160 if I were to swap instead? I suppose I could buy the 83mhz overdrive but I'm not exactly looking into spending a lot of money on something like that if I can just use an overclocked cpu.

Thank you!

Reply 2 of 29, by Charleston

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rmay635703 wrote on 2021-10-25, 22:51:

You can Bodge wire the multiplier pin to ground to force the 4x multiplier

My guess is something (cache, memory, vlb or timings) doesn’t like 40mhz

I can't seem to change my timings in BIOS, no clue why exactly. (AMI bios)

With cachechk I seem to get 11, 26, 93 on average throughout, can't check with clock 4x though.

Running the dx4-100 at 4x would make it run at 133mhz correct? I wonder if the bios just doesn't support it whatsoever and freaks out?

Reply 3 of 29, by jheronimus

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I have a fairly similar board (QDI V4P895GRN/SMT V1.1) and I've used it with AMD 5x86 at 40MHz, 0ws and tight cache/RAM timings without any issues. One thing that did give me issues on 40MHz was the disk controller. I've eventually settled on QDI QD6580 card and that thing worked great.

So I would look at the disk controller first: check its speed settings (some controllers have jumpers for bus speed), try using 1 wait state on the board, test with an ISA controller. If it does turn out to be the controller that gives you issues, that QD6580 is pretty common and I think is one of the best-regarded IDE controllers for VLB.

Also don't rely on the POST screen to check whether your CPU is set up correctly — I think it treats even 5x86 as DX4-S or something because it just doesn't know what 5x86 is. Maybe it's the same with DX4 overclocked to 120. CHKCPU is a better tool for that.

Last edited by jheronimus on 2021-10-25, 23:30. Edited 1 time in total.

Reply 4 of 29, by Charleston

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jheronimus wrote on 2021-10-25, 23:23:

I have a fairly similar board (QDI V4P895GRN/SMT V1.1) and I've used it with AMD 5x86 at 40MHz, 0ws and tight cache/RAM timings without any issues. One thing that did give me issues on 40MHz was the disk controller. I've eventually settled on QDI QD6580 card and that thing worked great.

So I would look at the disk controller first: check its speed settings (some controllers have jumpers for bus speed), try using 1 wait state on the board, test with an ISA controller. That QD6580 is pretty common and I think is one of the best-regarded IDE controllers for VLB.

Also don't rely on the POST screen to check whether your CPU is set up correctly — I think it treats even 5x86 as DX4-S or something because it just doesn't know what 5x86 is. May be it's the same with DX4 overclocked to 120. CHKCPU is a better tool for that.

Do I need a vlb controller? I'll check it out.

Reply 5 of 29, by jheronimus

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Charleston wrote on 2021-10-25, 23:30:

Do I need a vlb controller? I'll check it out.

My bad, I kind of assumed that you are already using one. Are you using an ISA controller? I don't think those can be affected by a 40MHz bus.

Reply 6 of 29, by Charleston

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jheronimus wrote on 2021-10-25, 23:32:
Charleston wrote on 2021-10-25, 23:30:

Do I need a vlb controller? I'll check it out.

My bad, I kind of assumed that you are already using one. Are you using an ISA controller? I don't think those can be affected by a 40MHz bus.

Yeah using ISA

Reply 7 of 29, by jakethompson1

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This explains how the Am5x86 behaves differently depending on whether the multiplier is 3 or 4: https://www.amd.com/system/files/TechDocs/19720.pdf
Basically, the CPUID changes such that if the multiplier is 3, it presents itself as an Enhanced Am486DX4, and if it is 4, it has a new CPUID for the 5x86.
So the question is whether your BIOS doesn't like the CPUID, or if the speed timing loop is very buggy such that something overflows when the speed is 133MHz.

You could experiment by setting it to 4x, but on a 25MHz bus. If it won't POST, it's probably the CPUID.
You could also try setting it to 3x, but on a 50MHz bus. If it posts (and will probably present as an Am486DX4-100 in POST), it's not the timing loop.
If you haven't seen AMISETUP, it may give access to hidden settings to control timings.

Edit: Also, I think the multiplier works so that a DX4 interprets 4x as 2x but an Am5x86 interprets it as 4x, or something like that.

Reply 8 of 29, by Anonymous Coward

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Set JP43 to the 2X setting to run am5x86 in 4X mode.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 9 of 29, by Charleston

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Anonymous Coward wrote on 2021-10-26, 02:58:

Set JP43 to the 2X setting to run am5x86 in 4X mode.

How does this work? Are there any other jumpers I'd have to change or just this one? When I tried 2x on the 100mhz dx4 I got the proper 66mhz so how does it do differently on the amd chip?

Reply 10 of 29, by jakethompson1

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Charleston wrote on 2021-10-26, 21:51:
Anonymous Coward wrote on 2021-10-26, 02:58:

Set JP43 to the 2X setting to run am5x86 in 4X mode.

How does this work? Are there any other jumpers I'd have to change or just this one? When I tried 2x on the 100mhz dx4 I got the proper 66mhz so how does it do differently on the amd chip?

That's how a DX4 interprets it. When the 5x86 was released, they redefined 2x as 4x. Similar to how some of the later Socket 7 Pentiums redefined some of the low multipliers as 3.5.

Did you try the things I suggested above with the 5x86? Like running it at 4x 25MHz? If that doesn't work, and it's a CPUID check, it's possible a patch of your BIOS could get around it.

Reply 11 of 29, by Charleston

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jakethompson1 wrote on 2021-10-26, 21:56:
Charleston wrote on 2021-10-26, 21:51:
Anonymous Coward wrote on 2021-10-26, 02:58:

Set JP43 to the 2X setting to run am5x86 in 4X mode.

How does this work? Are there any other jumpers I'd have to change or just this one? When I tried 2x on the 100mhz dx4 I got the proper 66mhz so how does it do differently on the amd chip?

That's how a DX4 interprets it. When the 5x86 was released, they redefined 2x as 4x. Similar to how some of the later Socket 7 Pentiums redefined some of the low multipliers as 3.5.

Did you try the things I suggested above with the 5x86? Like running it at 4x 25MHz? If that doesn't work, and it's a CPUID check, it's possible a patch of your BIOS could get around it.

Not quite, gonna try it tonight as I haven't had the time. Maybe it'll just work 🤣. As far as I'm aware all the revisions of this board support it except this one so it could be a bios issue. Will update when I get to it.

Reply 12 of 29, by jakethompson1

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Charleston wrote on 2021-10-26, 22:12:

Not quite, gonna try it tonight as I haven't had the time. Maybe it'll just work 🤣. As far as I'm aware all the revisions of this board support it except this one so it could be a bios issue. Will update when I get to it.

Can you dump your BIOS at some point? That board is confusing as it looks like there was a multitude of different hardware and BIOS revisions. Possibly one of the later BIOSes might solve your issue, but I'm wondering how your current one will play out inside PCem while messing with the emulated CPU type.

Reply 13 of 29, by Charleston

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jakethompson1 wrote on 2021-10-27, 00:48:
Charleston wrote on 2021-10-26, 22:12:

Not quite, gonna try it tonight as I haven't had the time. Maybe it'll just work 🤣. As far as I'm aware all the revisions of this board support it except this one so it could be a bios issue. Will update when I get to it.

Can you dump your BIOS at some point? That board is confusing as it looks like there was a multitude of different hardware and BIOS revisions. Possibly one of the later BIOSes might solve your issue, but I'm wondering how your current one will play out inside PCem while messing with the emulated CPU type.

Trying out your possible solutions right now. Will try to dump the bios sometime soon

Reply 14 of 29, by Charleston

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So it appears it just absolutely can not run anything more than 100mhz whatsoever. It either crashes on an overclocked dx4 100 while booting or it hangs on the wait screen after posting with the amd set to your settings as you had said. I also tried putting the amd in as a 120mhz with it set to 40 with a x3 and that didn't work either. How would I go about updating the bios (reflashing it or finding another ami for a future revision of this board)???

Edit: forgot to try 2x 25, will try that in a bit.

2x 25 didn't even post, black screen.

Reply 15 of 29, by jakethompson1

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Charleston wrote on 2021-10-27, 03:22:

So it appears it just absolutely can not run anything more than 100mhz whatsoever. It either crashes on an overclocked dx4 100 while booting or it hangs on the wait screen after posting with the amd set to your settings as you had said. I also tried putting the amd in as a 120mhz with it set to 40 with a x3 and that didn't work either. How would I go about updating the bios (reflashing it or finding another ami for a future revision of this board)???

Edit: forgot to try 2x 25, will try that in a bit.

2x 25 didn't even post, black screen.

And when you are using the Am5x86, you are using the P24D (WB) jumper settings?

Reply 16 of 29, by Charleston

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jakethompson1 wrote on 2021-10-27, 03:32:
Charleston wrote on 2021-10-27, 03:22:

So it appears it just absolutely can not run anything more than 100mhz whatsoever. It either crashes on an overclocked dx4 100 while booting or it hangs on the wait screen after posting with the amd set to your settings as you had said. I also tried putting the amd in as a 120mhz with it set to 40 with a x3 and that didn't work either. How would I go about updating the bios (reflashing it or finding another ami for a future revision of this board)???

Edit: forgot to try 2x 25, will try that in a bit.

2x 25 didn't even post, black screen.

And when you are using the Am5x86, you are using the P24D (WB) jumper settings?

Yeah, I'm just redoing everything through the manual. Amd doesn't get past wait screen when set properly, regardless of speed.

Edit: so it seems when set to amd settings after leaving it on long enough it's actually getting a c drive error?

Reply 17 of 29, by jakethompson1

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Charleston wrote on 2021-10-27, 03:40:
jakethompson1 wrote on 2021-10-27, 03:32:
Charleston wrote on 2021-10-27, 03:22:

So it appears it just absolutely can not run anything more than 100mhz whatsoever. It either crashes on an overclocked dx4 100 while booting or it hangs on the wait screen after posting with the amd set to your settings as you had said. I also tried putting the amd in as a 120mhz with it set to 40 with a x3 and that didn't work either. How would I go about updating the bios (reflashing it or finding another ami for a future revision of this board)???

Edit: forgot to try 2x 25, will try that in a bit.

2x 25 didn't even post, black screen.

And when you are using the Am5x86, you are using the P24D (WB) jumper settings?

Yeah, I'm just redoing everything through the manual. Amd doesn't get past wait screen when set properly, regardless of speed.

No difference with L2 (external cache) off? With DRAM wait states set to the highest? With L1 (internal cache) off? WAIT... is usually when the BIOS switches from the pessimistic settings burned into the BIOS ROM, to whatever you've configured in the CMOS setup.
If L2 off helps, something is too aggressive for your bus speed or cache chips. Likewise with DRAM wait states and your SIMMs. L1 off helping would indicate a problem with the L1 write-back stuff although usually it would get past that point and hang when booting from a floppy.

Reply 18 of 29, by jakethompson1

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Charleston wrote on 2021-10-27, 03:40:

Edit: so it seems when set to amd settings after leaving it on long enough it's actually getting a c drive error?

Interesting! since it's established you are using an ISA IDE interface and not VLB, CPU/bus speed shouldn't be a factor in that. Everything IDE is hooked up correctly?