That's great. You don't often see Pentium P54Cs clock into 286 territory. If you have a POD200MMX, you can get 1x multiplier (when its fan is disabled), and a 16 MHz Pentium that's even slower Your Speedsys benchmark should drop from 2.71 to below 2.0.

Some boards can use a reference clock of 7MHz for even more slowdown. And if the board is Turbo enabled, Speedsys can drop below 0.5 (with a K6-III+), which is Intel 8088 4. 77MHz territory.

I'm assuming this is being fed into the clockgen chip. Is there a way you could flip between the default crystal and your custom PLL as a kind of DIY deturbo mode?

@ jaws. Too bad, I got neither of those CPUs.

@ gdjacobs. You could switch original 14.3 MHz ref. crystal resonator to a lower freq. one, but a simple replacement won't do, because you would still need a 14.3 MHz clock for chipset or else whole timing will be off, also, software would report false CPU frequency because of that (TSC register issues).

Probably slowest P1 out there @ 8 MHz:

Cacheless config, FSB runs @ 5.35 MHz, PCI @ 2.68 MHz

Even with cache on, games like "Pinball Fantasies" are unplayable.

Neat... at these speeds, you could probably get away with no heatsink attached, eh?

Haha, yes, @ 3.4V and 8/25 MHz CPU barely reaches 40*C w/o heatsink, with fan on - it's at room temp.

SSTV2 wrote:

@ gdjacobs. You could switch original 14.3 MHz ref. crystal resonator to a lower freq. one, but a simple replacement won't do, because you would still need a 14.3 MHz clock for chipset or else whole timing will be off, also, software would report false CPU frequency because of that (TSC register issues).

That's 14.3 mhz in addition to the output of the clock gen?

Impressive! Have you tried some classic XT games to see if they're playable?

SSTV2 wrote:

Probably slowest P1 out there @ 8 MHz:

Cacheless config, FSB runs @ 5.35 MHz, PCI @ 2.68 MHz

That's very good. There was a slower P1 in this thread:
The IBM XT challenge: How slow can you go with i386+ (IA-32) CPUs?

The posters image links seem broken. IIRC, the Pentium was showing a Norton SI score of around 1.6 instead of 2.4 in the screenshot. Athough, there is written a Topbench score of 10. You should post in that thread for posterity, and also a Topbench score if possible.

Try a scrolling speed sensitive XT game like Striker from 1985 - it's a shareware DOS game, like a Defender/ Chopper Command clone. The scrolling speed and control is very CPU sensitive and requires an 8088@ 4.77MHz.

I have tried running few XT games today and most of them lagged 😀

"OFF ROAD" XT game, it has few selectable CPU optimisation options at startup:

Config: P1 @ 8 MHz, no cache, this time 32 MB FPM RAM with highest timing settings in BIOS. Here are few video demos with this P1:

XT mode for 4.77 MHz CPUs. Playable, but frame rate is not perfect, background can be seen constantly redrawing itself.

Turbo XT mode for 8 - 10 MHz CPUs. Even choppier.

Avion:

Topbench and System Info 6 results for j^aws:

8088MPH system test screen:

I can see that guy named Trodas at linked thread had dropped his P1 clock down to 10.7 MHz using same technique as myself (with the help of PLLs REF/2, REF/4, test mode setting).

SSTV2 wrote:

I have tried running few XT games today and most of them lagged :) [...] Topbench and System Info 6 results for j^aws: https:/ […]
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I have tried running few XT games today and most of them lagged 😀
[...]
Topbench and System Info 6 results for j^aws:

Thanks for the benches.

If you are finding the games 'lagging', it could be that you are running too slow. What video card are you using? Can you adjust your CPU speed with enough granularity and speedup? These benches are a guideline for equivalent system speeds, so can be somewhat off when using real world software.

I also tried various video cards to get better results, especially ones that have a CGA Mode.

j^aws wrote:

What video card are you using? Can you adjust your CPU speed with enough granularity and speedup?

Video card is S3 Trio64V+ with 1 MB EDO. Yes, I can technically increment/decrement CPU freq. in 100 KHz steps by using appropriate freq. reference resonators, but my goal is not to slow down P1 enough just so I could play old games. Originally I just wanted to see whether I could simply adjust FSB freq. by feeding external clock to CPU, but in the process discovered that this PLL had some neat capabilities 😀

Anyway, 8 MHz was the lowest freq. I could get a P1 running at, with a 8.9 MHz ref. crystal, system refused to post and next, slightly faster crystal I had, was a 10.7 MHz one, which I used to get these results. By some reason, I previously thought that 33 MHz freq. was the operating limit of a P1, but now it seems that even 8 MHz is not it.