I've definitely come across motherboard manuals with jumpering sections that know about the "P24D", but for a DX4, implicitly assume the DX4 is the write-through version. So I am curious which came first, at least in terms of motherboard designers knowing what to accommodate.

In addition to what you say about layout changes, there also plenty of boards offered in another (possibly OEM) variant where they simply omitted all the 3.3V components.

mkarcher wrote on 2025-04-19, 01:16:

I'd be surprised if a mainboard at the time the P24D was available at the market chose to omit 3.3V support

My SX955 is 89+93 and my DEC Venturis 4 BIOS is 1.02 12/23/94, but I don't know when the DEC Venturis 4 ver 1.0 was released.

jakethompson1 wrote on 2025-04-19, 01:26:

Implicitly assume the DX4 is the write-through version.

AFAIK the CPUID 0480 486DX4-100 are only WT and the CPUID 0490 WB, a CPUID 0490 in WT reports a CPUID of 0483.

Do CPUID 0490 support PSE (Page Size Extension) ?

red-ray wrote on 2025-04-18, 12:57:

Thank you, I tried a 27 Ω which gave 3.58 volts, booted into NT @ 100 MHz and started SIV. Next I switched the bus to 40 MHz, SI […]
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MikeSG wrote on 2025-04-18, 12:04:

Yes should be able to run a resistor there.

Thank you, I tried a 27 Ω which gave 3.58 volts, booted into NT @ 100 MHz and started SIV. Next I switched the bus to 40 MHz, SIV updated the speed to 120 MHz and a couple of seconds later I got a BSOD.

I guess I need to go for a higher voltage, but am reluctant to do this. I think I will get a Model 9 DX4-100 (SX209, SK896,SK096), if I can find one, and see if that does any better. The SX896 is listed as '89, '94, maybe it will do better, but I wonder if '94 is an error and thus far have been unable to find a picture of one to check.

Out of interest what does the AMD2X jumper do?

What kind of fan are you using and are you cooling the regulator?

red-ray wrote on 2025-04-18, 21:50:

I tried 3.70 and it was worse than 3.60 !, as soon as a switched to 40 MHz NT gave a BSOD. I also tried starting at 40 MHz, NT Loader started and got stuck before it got very far.

I switched back to 3.60 and when I switch to 40 MHz SIV updated the speed to 120 MHz and it ran for about 20 seconds before a BSOD.

The best resistor to use is a modern blue coloured metal-oxide resistor with 1% tolerance. Higher tolerance resistors may not cause the VR to respond to dips as fast. Don't know if it'll make a difference in reality but worth a try.

Cache RAM also needs to be 20ns or faster for 40Mhz.

MikeSG wrote on 2025-04-19, 08:48:

The best resistor to use is a modern blue coloured metal-oxide resistor with 1% tolerance. Higher tolerance resistors may not cause the VR to respond to dips as fast. Don't know if it'll make a difference in reality but worth a try.

Cache RAM also needs to be 20ns or faster for 40Mhz.

I don't have any with the correct value, only 22 + 47 + 150, would https://www.ebay.co.uk/itm/315020833497 be a good choice. Ideally I would get a box with a selection, but can't find an E96 one.

The system runs well with a AMD Am5x86 @ 160 MHz so I don't think the cache is an issue, the board is a https://theretroweb.com/motherboards/s/dec-ve … 86-433-ver.-1.0 and AFAIK it's the CY7C199 15PC chips.

rmay635703 wrote on 2025-04-19, 02:30:

What kind of fan are you using and are you cooling the regulator?

I does not have a fan, maybe some air from the CPU fan helps, either way it's easily cool enough to touch. I guess I could fit a bigger one, what you you recommend please? How about https://www.ebay.co.uk/itm/286041915388?

My DX2-66 SX911 just arrived and it happily runs @ 80 MHz, the SIV [Save Local] took 4:50 and for most of that the CPU was 100% loaded.

red-ray wrote on 2025-04-19, 11:43:

My DX2-66 SX911 just arrived and it happily runs @ 80 MHz, the SIV [Save Local] took 4:50 and for most of that the CPU was 100% […]
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My DX2-66 SX911 just arrived and it happily runs @ 80 MHz, the SIV [Save Local] took 4:50 and for most of that the CPU was 100% loaded.

I'm not sure whether your enhanced i486 DX2-66 can run faster clock frequencies too.
5 Volt and 80 MHz will need a good cooling though.

I'd play with that processor.
First I try to run it with 25 MHz and try the DX4 jumper position to change the multiplicator and see whether it gets 75 MHz.
Then I'd try whether it will operate at 3.3 Volt too and probably go for 100 MHz then. 😀
If it runs at 3.3 Volt, it maybe a failed DX4-100 or a it was relabelled due to commercial issues.
How have you detected the cache size?

MikeSG wrote on 2025-04-19, 08:48:

The best resistor to use is a modern blue coloured metal-oxide resistor with 1% tolerance.

The blue coloured metal-oxide resistors I ordered along with a bigger heatsink and and some 2 way female PCB header sockets have arrived so I though I would try 3.80 volts.

I used a 68 Ω resistor with resulted in 3.78 volts, it will post @ 120 MHz, but won't boot NT and if I boot @ 100 MHz the instant switch to 120 MHz I get a BSOD.

I plan to continue looking for a later DX6-100, the one that supports write-back, and if that does any better.