I may be lucky then. I got it up and running at 120Mhz only needed to decrease the RAM timing to Fast (instead of Fastest). What are you suggestions feipoa on the longevity of this setup? I believe the chip is running at 3.3/3.45 or at the most 3.6V (I don't know exactly because of the missing motherboard manual) with a standard heatsink and fan (currently without thermal compound). Would it be OK to use it for an extended period of time with these settings? It performs quite well anyway, 3DBench gives me exactly 100 points (or cannot display more) and Speedsys shows these results.

Thanks debs3759 but the battery seemed to be functional (it stored 4.1V fully charged and held the CMOS data) the problem is it caused some issues with the clock so it had to go. It might be a faulty one though so I will experiment first with external battery and then maybe add a new internal one.

If you are overclocking without thermal paste, you processor is at high risk of overheating, as the heatsink won't make a perfect seal on the CPU, so won't keep it as cool as it needs. I highly recommend keeping it turned off, or at least not thrashing the system, until you are able to apply some reasonable paste.

What about Cyrix (ST and TI built it, too) Cx486DX2 ? So far, at least this CPU (Cx486DX2-66 and Cx486DX2-80) have a lot of additional features, e.g. Write Back Cache (which was implemented for Intel 486DX CPUs later, with the 486DX4-100, also with the AMD 486DX-133). As far as I can see, some other CPU features found in the 5x86 are also implemented, so for socket 3/486 mainboard owners, even for the 5V only core voltage ones, this would be interesting/it would be interesting to enable these additional features. My own 486 mainboard does NOT offer any Cyrix CPU option in the BIOS, but at least two utilities support the Cx486DX also (ctchip34 and "wbon"/"wboff" found at https://github.com/karcherm/cx486wb). Unfortunately Peter Moss 5x86 tool are *not* working, also because it expects only a 5x86, regardless of the fact, that a lot of CPU registers/SMM storage addresses are the same.
Does anybody know an other Cx486DX compatible tool (and no, Cx486DRx tools are also NOT working) ?

Nice document!

Thank you!

So I got a 5x85, stepping 0, revision 5, more specifically a 100QP variant (The laptop chip, soldered on an interposer).
For all test, the chip is overclocked to 120Mhz.
The motherboard is an Acer A1GX-2 (with Ali 1429G chipset) with registry patched for fastest memory and 0 WS for the cache.

On DOS I made two config :
* All enabled except LOOP
* All enabled except BTB (some DOS application don't seam to like BTB)

On windows, I run it with everything except LOOP, RSTK, BWRT, and DTE.
You are right about stability going away (probably with heat?). But after testing I noticed that I don't need 3.85v. The chip is totally happy with 3.45V.
Once I switched to 3.45V, the stability issue went away (I think). So far the system is playing 320kbps MP3 non stop since about an hour (With 3.85V, it fail after less than 10 minutes).
I can still make it crash if I load the CPU to 100% and do a LOT of task switching (upload stuff by FTP while playing the 320k MP3).

From what I read before I did not expect that chip to be fine with 3.45V while overclocked at 120Mhz, but I suppose the QP variants might be higher bins as they are supposed to be in a (usually badly cooled) laptop.

Chip won't post at 150Mhz (at least not at 3.45v or 3.85v).
There is one issue on that motherboard with this CPU running 120Mhz : The board don't set the proper AT bus clock by default with mean one time out of two, pressing a key will <beep> in the BIOS / bootloader / dos boot menu. Also the bootloader will fail to initialize A20 and crash one time out of two.

But once it boot, chipset registers get patched, including AT bus, the issue go away. I use fastreboot that come with QEMM so registers stay patched on reboot (if I want to go in the BIOS without having a fight with the keyboard)

Where did you find your CPU? It is hard to find these QFP to PGA variants.

The Vcc rail on these PCBs is kind of noisy. You could try adding some caps to reduce this some. Might help with OC, might not. I used 100 nF for the smaller caps and 10 uF for the larger ones.

Here's mine:

feipoa wrote on 2023-06-22, 20:04:

Where did you find your CPU? It is hard to find these QFP to PGA variants. […]
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Where did you find your CPU? It is hard to find these QFP to PGA variants.

The Vcc rail on these PCBs is kind of noisy. You could try adding some caps to reduce this some. Might help with OC, might not. I used 100 nF for the smaller caps and 10 uF for the larger ones.

Here's mine:
Cyrix_5x86-120_QFP_1.JPG

Thanks for the tip, will look for capacitors.

I got the CPU in the local online market, there is someone who regularly sell "collector / dead" CPU. for 10-15$ a pop. Also got a TI486 DX4 there.
By "dead" he mean he can't test them 😀
I look every weeks to see what "dead" chip he his selling, Usually range from 486 to P4/Athlon.