VOGONS


First post, by mpe

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I was wondering what would be the perfect upgrade for my 386 system. It has 386DX-40 soldered-in, an unpopulated 386 PGA socket and a socket for either 387 FPU or 486 PGA or (might take a Weitek as well - not sure) and even might come with a soldered-in 486SX PQFP. A plenty of options...

But how about hacking a Pentium in?

Obviously, the first hurdle is that my motherboard doesn't have a Socket 2 that is required for Pentium Overdrive, let alone Socket 3. It doesn't even have a Socket 1. It only has a hybrid socket that can hold either 486 PGA cpu or 387 FPU (in case a 386DX CPU is installed).

DSC_9228-scaled.jpeg

However, looking at the Pentium Overdrive pinout the extra row of pin doesn't seem to be at all essential. There is a number of extra power points and some signalling pins to support L1 cache coherency when using write-back. Nothing too much to worry about. Conveniently, the PODP doesn't use 486 WB cache controls like DX4 or 5x86 does, so the WB/WT pin remains floating and no need to configure the board for P24T like you do it on normal 486 motherboards.

I couldn't be bothered to set any jumpers and left the board configured for 486SX.

Screenshot 2021-02-20 at 17.42.13.png
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As the socket wasn't designed for much wider PODP it helps to use a PGA169 socket to lift the wide CPU up a bit so that the unused row of pins doesn't interfere with jumpers and other components that are in surrounding of the CPU socket.

Resulting combo looks quite weird:

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However, who cares about how it looks from bottom. It looks OK when installed.

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And in less than 7 minutes of terror, believe or not, it ran! The Mr. BIOS on my board is quite confused and reports the CPU to be 586SX, which I like.

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Obviously slower than in newer 486 boards, the L1 runs in write-through mode, but it runs just fine.

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I thought there must be a reason for the PODP having the extra power pins. Just to play it safely I configured maximum power on my ATX2AT device and set the frequency to 25MHz. However, it turns out that idling only @ 3.6A - not too far from ordinary 486DX.

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I wouldn't probably run it for extended period of time, but it was a successful experiment showing that all those extra pins on the PODP are only optional.

Having a Pentium FPU this "386" would beat even some systems from World's fastest 486 thread in Quake 😀

Last edited by mpe on 2021-02-20, 20:12. Edited 2 times in total.

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Reply 1 of 30, by aaronkatrini

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Expecting to be click-baited from this thread, I thought what the heck... I'll click anyway...but to my surprise you managed to pull it off!! Nicely done 😁
I'm curious how does it compare to a 40mhz 386?

Reply 3 of 30, by mpe

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kolderman wrote on 2021-02-20, 20:00:

Cool. But what on earth is that AT2ATX device? Has it got a live display? That's really cool. Where did you get it from?

Yes.

ATX2AT Smart Converter - Live on Kickstarter!

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Reply 5 of 30, by Brightraven

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Did your board originally come with an AMI BIOS? Could you dump the MR BIOS?

I have this board (with only the 486 PGA socket on board) but it has a weird performance issue when benchmarking DOOM as it would slow down or lock up. I eventually found out it slows down if you don't have a battery installed, no idea why having no battery would cause performance to suffer unless it somehow does not save all the BIOS settings or corrupts them after a warm boot.

Edit: Just checked my board the main chip is a 495SLC, not an 495XLC. Yet both boards are identical.

Last edited by Brightraven on 2021-02-20, 20:54. Edited 1 time in total.

Reply 6 of 30, by kolderman

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Brightraven wrote on 2021-02-20, 20:49:

Did your board originally come with an AMI BIOS? Could you dump the MR BIOS?

I have this board (with only the 486 PGA socket on board) but it has a weird performance issue when benchmarking DOOM as it would slow down or lock up. I eventually found out it slows down if you don't have a battery installed, no idea why having no battery would cause performance to suffer unless it somehow does not save all the BIOS settings or corrupts them after a warm boot.

Does the clock need the battery?

Reply 7 of 30, by Carlos S. M.

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Next challenge: boot Windows XP on it 🤣

What is your biggest Pentium 4 Collection?
Socket 423/478 Motherboards with Universal AGP Slot
Socket 478 Motherboards with PCI-E Slots
LGA 775 Motherboards with AGP Slots
Experiences and thoughts with Socket 423 systems

Reply 10 of 30, by mpe

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Brightraven wrote on 2021-02-20, 20:49:

Did your board originally come with an AMI BIOS? Could you dump the MR BIOS?

Frankly, I am not sure which BIOS is native for this motherboard. It is quite beaten-up board. It came to me with Award BIOS which I only had problems with (HIMEM.SYS failing, L2 cache not working). It was in EPROM with "Award 386DX BIOS sticker", but might or might not be the original. The board clearly had a battery accident and some tantalum filtering caps are missing (likely after an explosion - I shall be replacing them) so someone might have been working on it.

Filename
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I programmed this random Mr.BIOS I found online which seems to be working better.

Filename
O495XLC.ROM.zip
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Last edited by mpe on 2021-02-20, 22:17. Edited 1 time in total.

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Reply 11 of 30, by mpe

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quicknick wrote on 2021-02-20, 21:50:

Next step: insert another PGA237 socket between the PODP and the PGA169, connect all appropriate pins to VCC and GND, set bus to 33MHz, enjoy! 😀

I 've been thinking about this. However, it would probably only make sense with external Molex adaptor as the point of these extra pins is to protect the motherboard - not the CPU. Internally all those pins are connected in the CPU anyway.

I am thinking if Opti 495XLS supports write-back CPUs (can't find the datasheet). Then it would be possible to build an interposer that would route P24T signals to 486 (P24D) and get write-back cache working...

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Reply 12 of 30, by Deksor

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According to the POST string, this bios was made for a Jetway/J-mark motherboard (10/01/93-MX83C305/306-213b0J10-00)
But your board is clearly a DataExpert : http://www.win3x.org/uh19/motherboard/show/5531

Sadly we don't have any bios for it
(Hi bancho ! Can you help ? ^^)

Bancho wrote:

Trying to identify old hardware ? Visit Ultimate Hardware 2019 - Project's thread The Ultimate Hardware 2019 (UH19) project- a stason.org/TH99
alternative

Reply 13 of 30, by Bancho

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Deksor wrote on 2021-02-20, 22:35:
According to the POST string, this bios was made for a Jetway/J-mark motherboard (10/01/93-MX83C305/306-213b0J10-00) But your bo […]
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According to the POST string, this bios was made for a Jetway/J-mark motherboard (10/01/93-MX83C305/306-213b0J10-00)
But your board is clearly a DataExpert : http://www.win3x.org/uh19/motherboard/show/5531

Sadly we don't have any bios for it
(Hi bancho ! Can you help ? ^^)

Bancho wrote:

Sure, Mines an AMI bios. I'll get it dumped asap.

Last edited by Bancho on 2021-02-20, 23:09. Edited 1 time in total.

Reply 14 of 30, by Brightraven

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mpe wrote on 2021-02-20, 21:54:

I programmed this random Mr.BIOS I found online which seems to be working better.

O495XLC.ROM.zip

Thanks, mpe I will try the MR BIOS when I get the chance.

Reply 18 of 30, by Sphere478

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I love everything about this post 🤣

look! a program Jan wrote! I'm running two of his modded bios

tell me about that power supply adapter thing? why does someone need a atx to at adapter with a control board in it? what does it do?

edit: found it, neat!
https://www.kickstarter.com/projects/doctb/at … -retro-hardware

I didn't even realize that they had dropped -5v from 24 pin standard. turns out the psu I am using with a AT adapter actually is a 24 pin but does have the -5 so mine is fine confirmed -5 with volt meter. learned something though

🖥Fastest socket 7 build on a 430tx chipset
🖥Dual socket 7 build

Reply 19 of 30, by mpe

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Performance @ 83 MHz has been rather disappointing so far.

The CPU performance is amazing, but L2/mem remains still @ mediocre 386 levels which is holding back the performance. Hope it can be tuned slightly higher. So overall it is a hair better than a 486DX2-66 on average 486 board.

Getting about 9fps in Quake and 27fps in Doom test (Phil's benchmark pack versions). The RAM speed is slow even to 386 standards.

I cannot tune L2 cache to run at 2-1-1-1. Will need to solder-in the missing filtering caps and use the native AMI BIOS (if you guys can post it it would be great).

Looks like at 83 MHz the PODP draws only about 4A at when running Quake, so not a big deal.

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