First post, by Sphere478

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Spawned from a fun discussion on discord the thought experiment was put forth how slow can you go?

I think 7mhz is possible, but I don’t have the hardware to do it.

I can pull off 50mhz though.

Let’s see those underclocks!

The theorized secret sauce:

Mobo: Lucky Star 5V-1A
Lucky Star 5V-1A / 5V-1B review - scaling form XT levels to K6-II levels

The cpu:
Cyrix 366gp
(Solved) Cyrix 400gp/366gp multiplier settings

This mobo supports 6-8mhz fsb
And the cpu supports 1x multipler

Bonus points: make it to windows for a screenshot, and do it without a heatsink!

Sphere's PCB projects.
Sphere’s socket 5/7 cpu collection.
SUCCESSFUL K6-2+ to K6-3+ Full Cache Enable Mod
Tyan S1564S to S1564D single to dual processor conversion (also s1563 and s1562)

Reply 1 of 6, by Tetrium

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Something similar was done years ago using Socket 3 and a POD on an ASUS board.

Whats missing in your collections?
My retro rigs (old topic)
Interesting Vogons threads (links to Vogonswiki)
Report spammers here!

Reply 2 of 6, by dionb

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Some So5 boards had *very* low FSB options (as they were basically using 486 PLLs). One I had myself: Spring Circle P5C01. It had options down to 16MHz. That might not sound slower than the other options but...

This is a board with SiS 5511 chipset and a 6205 VGA chip using UMA. So the memory bandwidth with 16MHz FSB is shared between CPU and VGA. It was *SLOOOOOW*, running Windows XP not quite as slowly as on my Pentium Overdrive I was running at 16MHz (16MHz 32b bus x 1 by removing fan), but being in same ballpark.

Reply 3 of 6, by Repo Man11

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Ugh, sounds like torture to me. I recently used my TXP4 to test a K6 300 I have to make sure it works. And it does, but the difference between 6x83 with 256K L2 cache and 4.5x66 with only the motherboard's 512K L2 was night and day.

"Everyone is ignorant, only on different subjects." - Will Rogers

Reply 5 of 6, by zyga64

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Shuttle HOT-541 (430 FX) and HOT-553 (430 HX) have very flexible PLL. You may go down to 16MHz FSB.
Below is table (frequencies are measured with osciloscope). At 16MHz FSB motherboard was still able to boot-up with PCI Trio64V+
This is my HOT-553 with Pentium 120 (CPU multiplier set up as 1.5), I also have Cyrix 6x86 so I may try with 1x multiplier as well (in the future).


  • hot_553_fsb.png
    File size
    18.51 KiB
    File license
    Public domain

1) VLSI SCAMP /286@20 /4MB /CL-GD5422 /CMI8330
2) i420EX /486DX33 /16MB /TGUI9440 /YMF718+GUS
3) i440BX /P!!!750 /256MB /MX440 /SBLive!+Vibra16s

Reply 6 of 6, by BitWrangler

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I just had a thought... while Cyrix 6x86 and MII class might have a functional 1x multiplier (Implemented multis can vary with time, fab, and OEM version, apparently identical ST/IBM/Cyrix versions of same model have shown differences.) ... it might not be slower than a P54 or P55 at 1.5x ... because that core is particularly fast on 16 bit code... anything you need about "486" fast is probably mostly 32bit by then... anything you want TurboXT/286 fast is probably 16 bit code. It might get brought more into line with L1/L2 kneecapping, as then the 50% loss of memory speed is gonna show.

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.