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160mhz CPU on VLB Motherboard ?

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First post, by Intel486dx33

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Time for a New Build.
I have this Socket-3 Adapter that allows for an AMD 5x86 CPU to run at 133mhz maybe even 160mhz ?

Would this be okay to put onto a Gigabyte GA-486 VLB Motherboard ?

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Reply 2 of 21, by Chkcpu

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This could work fine if your board has a SiS471 chipset. Which GA-486 VLB board do you have exactly and what is the present BIOS version?
So I can check your BIOS for Am5x86 support.

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The Unofficial K6-2+ / K6-III+ page

Reply 3 of 21, by Intel486dx33

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Chkcpu wrote on 2022-05-06, 11:33:

This could work fine if your board has a SiS471 chipset. Which GA-486 VLB board do you have exactly and what is the present BIOS version?
So I can check your BIOS for Am5x86 support.

I’ll have to set it up and check. But I am pretty sure it only officially supports up to the 486dx4-100.

Reply 4 of 21, by Disruptor

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Intel486dx33 wrote on 2022-05-06, 15:50:

I’ll have to set it up and check. But I am pretty sure it only officially supports up to the 486dx4-100.

Which 486DX4?
There are so many of them!

Intel? AMD? Cyrix?
Write-Through? Write-Back?

Reply 5 of 21, by Intel486dx33

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Here is a link to the manual

GA-486-VS
http://www.motherboards.org/files/manuals/47/486vs8a.pdf

Looks like the motherboard by itself can do ( 3 x 40mhz = 120mhz )
But I am not sure about ( 4 x 40mhz = 160mhz )

Reply 6 of 21, by Disruptor

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According to the manual you don't need that interposer. The board itself supports 3.3 V CPUs. The Am 5x86 should interprete 2x as 4x, so 4x40 may not be a problem.

According to L1 write back CPUS:
¨P24D is suppored from PCB REV.8A & BIOS July, 12 1994 or later."

Please check your mainboard & bios for revision and date.

Reply 9 of 21, by TheMobRules

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The jumper settings for an AMD 5x86 should be the same as those for an Intel DX4, with the exception of the multiplier that Disruptor mentioned above + the L1 WB cache.

I have a GA-486VS rev .8a (using latest BIOS) and was unable to run an Intel DX4-WB with L1 in write-back mode. A P24D (DX2) works fine with L1 WB but with the DX4 the board fails to post as soon as I connect the jumper that pulls the WB/WT# line high (the HITM# and INV lines seem to be connected properly). Maybe Jan (@Chkcpu) can shed some light on this as he has done some very detailed research about the 5x86 in his page.

I'm also attaching the latest version of the BIOS provided by Gigabyte.

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    VSFBI.ZIP
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    Fair use/fair dealing exception

Reply 10 of 21, by Intel486dx33

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Do you mean this jumper circled in red ?

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Reply 11 of 21, by Cuttoon

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Apart from those bios, cache, chipset issues - there are actually some VLB boards around with native 3 V support.
I'd assume it's less common to use these high end CPUs on them mainly because a PCI board will make more of their potential...
But, fingers crossed this one will work!

I like jumpers.

Reply 12 of 21, by Chkcpu

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I agree that this GA-486VS Rev. 8a board is a good candidate for the Am5x86-P75 CPU. With its P24D (i486DX2WB) and DX4-100 support, it has proper hardware support for all 5V and 3.3V socket 3 CPUs.
The culprit however is the BIOS.

@TheMobRules, thanks for the latest official BIOS for the 486VS.
I’ve checked this Rev.A BIOS and as expected from a Nov. 1994 Award BIOS, it has P24D and DX4 support, including L1 cache WB support for Cyrix 486DX(2) and P24D.
But the iDX4WB, Enhanced Am486DX4, Am5x86-133 and Cyrix 5x86 are not supported. 🙁

I have 2 solutions for this BIOS issue:
1) Use an up-to-date SiS471 BIOS from another SiS471 board and adapt it for the 486VS
2) Patch the original Rev.A BIOS for Am5x86 support.

The first option is usually the quickest and most optimal solution. I’ve done this successfully on several other SiS471 boards.
The second solution is a lot more work, but may be needed if the first doesn’t work.

@Intel486dx33, I expect to have a BIOS update ready for you in 1 or 2 days.

Jan

CPU Identification utility
The Unofficial K6-2+ / K6-III+ page

Reply 13 of 21, by Intel486dx33

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Chkcpu wrote on 2022-05-08, 11:33:
I agree that this GA-486VS Rev. 8a board is a good candidate for the Am5x86-P75 CPU. With its P24D (i486DX2WB) and DX4-100 suppo […]
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I agree that this GA-486VS Rev. 8a board is a good candidate for the Am5x86-P75 CPU. With its P24D (i486DX2WB) and DX4-100 support, it has proper hardware support for all 5V and 3.3V socket 3 CPUs.
The culprit however is the BIOS.

@TheMobRules, thanks for the latest official BIOS for the 486VS.
I’ve checked this Rev.A BIOS and as expected from a Nov. 1994 Award BIOS, it has P24D and DX4 support, including L1 cache WB support for Cyrix 486DX(2) and P24D.
But the iDX4WB, Enhanced Am486DX4, Am5x86-133 and Cyrix 5x86 are not supported. 🙁

I have 2 solutions for this BIOS issue:
1) Use an up-to-date SiS471 BIOS from another SiS471 board and adapt it for the 486VS
2) Patch the original Rev.A BIOS for Am5x86 support.

The first option is usually the quickest and most optimal solution. I’ve done this successfully on several other SiS471 boards.
The second solution is a lot more work, but may be needed if the first doesn’t work.

@Intel486dx33, I expect to have a BIOS update ready for you in 1 or 2 days.

Jan

Thanks, but no rush. I don’t have an eprom programmer device anyways.
I will probably just stick with the Intel 486dx4-100 which what I currently have in this motherboard.

Reply 14 of 21, by TheMobRules

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Chkcpu wrote on 2022-05-08, 11:33:

1) Use an up-to-date SiS471 BIOS from another SiS471 board and adapt it for the 486VS

Can you provide some details on what kind of things you tweak in order to "adapt" a SiS471 BIOS to work on a different board? I have previously used an Award BIOS from the ASUS VL/I-486SV2GX4 on an MSI MS4132G (which has an awful AMI BIOS that severely hinders RAM and cache performance) and it seemed to work fine without any patches, getting about the same performance and CPU support as in the ASUS board. But I didn't do extensive testing so there may be incompatibilities I'm not aware of, and I wonder what kind of things would change in the BIOS between these 471 boards as they're all very similar in terms of layout, components, etc.

Chkcpu wrote on 2022-05-08, 11:33:

@Intel486dx33, I expect to have a BIOS update ready for you in 1 or 2 days.

I'm interested as well! Even if it is just to run an Intel DX4WB with L1 in write-back mode on the Gigabyte board.

BTW, your thread about Award BIOS modding is an invaluable resource. I wish it was possible to do it on 486 (and earlier) AMI BIOSes as well, as some of those are really subpar compared to the Award ones.

Reply 15 of 21, by Chkcpu

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Intel486dx33 wrote on 2022-05-08, 21:57:

Thanks, but no rush. I don’t have an eprom programmer device anyways.
I will probably just stick with the Intel 486dx4-100 which what I currently have in this motherboard.

An Intel DX4-100 is a good choice for this board as well and, as Disruptor pointed out, the Rev. 8a board has support for 3.3V CPUs so you don’t need the interposer for a DX4 either.

I did finish the GA-486VS BIOS update anyway, so you have a good BIOS for future CPU upgrades.
Attached below is this updated GA-486VS BIOS 11/28/95-SIS-85C471B/E/G-2C4I9G01-00 that I’ve named Rev. J.1.
This BIOS supports all socket 3 CPUs, including the Cyrix 5x86 and Am5x86 in L1 cache WB and x4 multiplier mode. Its also free of the year 2094 and 2GB HDD display limit bugs and supports drives up to 8GiB.

Note that this BIOS shows the “L1 cache: WB/WT” option in the BIOS Setup only for the P24T and for Cyrix CPUs.
For the P24D (486DX2WB), iDX4WB, Am486DX4WB and Am5x86, this 11/28/95 BIOS can detect if these CPUs are in WB mode and programs the chipset registers accordingly. It then hides the “L1 cache: WB/WT” option, because user interaction for this automatic function is not required.
So, you only have to find the correct jumpers for L1 WB mode and the BIOS takes care of the rest. 😉

Anyone who wants to try this BIOS update on their GA-486VS, please do so and let us know how it works.

Jan

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  • Filename
    GA-486VS-Rev_J.1.zip
    File size
    45.22 KiB
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    4 downloads
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    GA-486VS Rev. J.1 BIOS
    File license
    Public domain

CPU Identification utility
The Unofficial K6-2+ / K6-III+ page

Reply 16 of 21, by Chkcpu

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TheMobRules wrote on 2022-05-09, 01:34:

Can you provide some details on what kind of things you tweak in order to "adapt" a SiS471 BIOS to work on a different board? I have previously used an Award BIOS from the ASUS VL/I-486SV2GX4 on an MSI MS4132G (which has an awful AMI BIOS that severely hinders RAM and cache performance) and it seemed to work fine without any patches, getting about the same performance and CPU support as in the ASUS board. But I didn't do extensive testing so there may be incompatibilities I'm not aware of, and I wonder what kind of things would change in the BIOS between these 471 boards as they're all very similar in terms of layout, components, etc.

The adaptations I do when using a BIOS from another board with the same chipset, are part cosmetic, part functional, and are mainly done with Award’s Modbin tool.
I’m using my trusty old Win7 32-bit laptop for that, so I can open two cmd windows next to each other. One for the original and the other for the new BIOS.

Chipset options.PNG
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Chipset options.PNG
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161.27 KiB
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Modbin Chipset options
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With the cosmetics I try to let the new BIOS not look to different from the old one, with regard to placing and wording of BIOS options. But I always enable functional hidden options, like those for DRAM and Cache timings.
For the functional part, I check the Chipset registers defaults. When I see a distinct difference, like reg 61h in the example below, I try to find the meaning of the bit definition. Luckily for the SiS471 we have the datasheet. When the difference is justified, like a less conservative waitstate setting, I leave it in.

Register defaults.PNG
Filename
Register defaults.PNG
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152.83 KiB
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201 views
File comment
Modbin Regs defaults
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Public domain

Also the DRAM/Cache timing Auto Table settings for each supported FSB speed are worth checking. With the Tab key, you can step through each speed setting for one- or two banks of RAM.

Auto Table.PNG
Filename
Auto Table.PNG
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140.79 KiB
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201 views
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Modbin Auto Table
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Public domain

Finally, I change the manufacturer ID of both ID-strings to match it with the ID of the original BIOS, and correct the E-F page checksum at offset FFEEh (64KB BIOS) or 1FFEEh (128KB BIOS).

ManufacturerID.PNG
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ManufacturerID.PNG
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21.91 KiB
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201 views
File comment
Hexeditor BIOS-ID
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Public domain

What can I say, I’m a sucker for detail. 😉

Jan

CPU Identification utility
The Unofficial K6-2+ / K6-III+ page

Reply 17 of 21, by TheMobRules

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This is awesome Jan, many thanks.

I will give your BIOS a go on my GA-486VS tonight, at some point I had mapped all the required L1 WB jumper connections for this board when I attempted to run my iDX4WB but even if I'm unable to find my notes it should only take me a while to trace the CPU/chipset pins to the appropriate jumpers.

I will report back with my findings.

Reply 18 of 21, by Intel486dx33

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Sorry guys, I had to put this motherboard away. I could not get it to work with 3v CPU’s.
It would only work with 5v CPU. I was able to get it to work with the CPU adapter and an AMD 5x86 -133 @ 132mhz.
But the bios would see the CPU as a 486dx2@132mhz.

So this motherboard has a cache problem were it only works with 64kb setting.
I think there is a bad cache module socket that needs repair.

And there is the problem with 3v CPU’s not working.

So I decided I don’t have time or proper equipment to repair it right now.
Maybe in the future. I need a desolder gun, eprom programmer, and heat gun to flux solder.

Last edited by Intel486dx33 on 2022-05-10, 16:40. Edited 1 time in total.

Reply 19 of 21, by TheMobRules

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I am pleased to report that Jan's modded BIOS did the trick. I've tested with an iDX4WB-100 and a 5x86-133, both are detected properly and work with L1 cache in WB mode. Also, the performance is on par with the fastest VLB boards (not sure why Gigabyte crippled their own board with the conservative settings).

For future reference, here are the settings for both of these CPUs. Just follow the manual for an Intel DX4 and set these additional jumpers:

  • JP21: close 1-2 (this sets the hardware trap for WB L1 of the chipset)
  • JP24: close 2-5 (to connect HITM# line from the CPU to the chipset) and 3-4 on the 5x86 only (it sets the DX4 multiplier to x2, which is x4 for the 5x86)
  • JP30: closed (to pull the WB/WT# line high, enabling WB mode)

Note 1: there is no jumper for the INV pin, it seems to be permanently connected to Vcc, no jumper for CACHE# either
Note 2: if you set L1 to WB mode, DRAM Write Burst option in the BIOS needs to be disabled, otherwise it will hang when trying to boot MS-DOS

Also, I'm attaching a rev. J.2 of Jan's BIOS where I set bits 1, 2 of register #72 to 1 in order to enable L2 to use 7 tag bits + 1 alter bit in one SRAM chip when set to WB. This increases memory performance considerably, and is a known tweak for most SiS471-based motherboards (there are a few threads about this in the forum).

L2WB_DirtyTag.png
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L2WB_DirtyTag.png
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Thanks again, Jan!

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  • Filename
    486VS-J2.zip
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    44.73 KiB
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    4 downloads
    File license
    Fair use/fair dealing exception