Lets start with Am486 DX4-120 with 8k WB cache. Unfortunately I forgot to run speedsys twice so I only have 100 MHz WB results. I wanted 100 MHz WB and WT results for better comparison with Intel DX4.
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Clock for clock this is the slowest CPU I tested. At 100 MHz with 8k WB cache it is slower than Intel with 16k WT cache. At full 120 MHz it is only a little faster than Intel CPUs at 100 MHz. But even at full 120 MHz it consumes less power than Intel CPUs, thanks to 500nm core (Intel has 600nm) and of course half cache means less transistors to produce heat. AMD also has 500nm core with 16K cache but it is supposed to be rare. More likely AMD DX4 CPU with 16k cache is actually 350nm 5x86 core and that might be a bad thing because it has a problem when disabling L1 cache.
But nothing wrong with this CPU, no issues or crashes at all. It is simply slow and needs the extra 20 MHz to keep up with 100 MHz Intel.
Next the original CPU from this system, Intel DX4-100 with WT cache.
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As I mentioned on some previous post, to be able to slow down this for most problematic OPL-games, de-turbo (1/3) or disabling L1 alone is not enough. Both are needed and that makes the system much slower than necessary. I started tests with this CPU and was really happy that the new de-turbo 2/3 -mode doubled the speed over the previous 1/3+L1D combination and was slow enough for Cycles and likely other problematic games as well.
When testing the other CPUs it came clear that for ALL the other CPUs, even the overclocked ones, it is enough to disable L1 alone. Now how is that possible? I blame chipset. Motherboard has jumpers JP5 and JP6 (hardware trap) that are used to group different types of CPUs. In first group there are all traditional SX, DX, SX2, DX2 and DX4 CPUs with only WT cache. Next group has Cyrix CPUs and last group is for Intel and AMD WB CPUs. So from all tested CPUs this is the only one from group 1. I'm sure the chipset does something differently in this group. This also means that if you have motherboard with some other chipset, then likely the usable slowdown options are different. But most likely L1D + de-turbo combination is enough.
But this was not the only issue with this CPU. It overheated during the tests. At one point system crashed. I rebooted and it crashed very soon again. Heatsink was quite warm but not too hot to touch. Then I turned the PSU as seen on the picture: it sucks air through the CPU heat sink. No problems after that. This CPU was tested with different heat sink than the other CPUs. This one has original heat sink glued on. I don't know what glue is it or how good is it when it has gotten old. Maybe it acts more as thermal insulator than thermal conductor by now.
So I'm going to replace the CPU from this system. The latter problem could be resolved by removing the heatsink and glue but I can't do anything with the L1D + de-turbo issue. Chipset simply handle it better for other CPUs. But otherwise Intel DX4-WT is fast and good CPU but it does use more power than any other non-overclocked CPU and so it produces more heat.
Next we have Intel DX4-WB:
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Little faster than WT model and consumes little less power and L1D alone is enough for the problematic OPL games. So in every way better than WT model. But at 120 MHz this is the second fastest CPU I tested. Cyrix has faster FPU but that is not really needed for 486 games and Cyrix does win more tests but loose some. Overclocking does increase power consumption and heat but only little above Intel DX4-100 WT. Very good CPU. I was surprised how good it does at 120 MHz.
I did have one crash during the tests but I think it was my fault. This was the second CPU I tested and I sometimes left the OPL3 in error state after unsuccessful test with Cycles and Indy3. I then stopped doing that (I reset the system before starting new tests if OPL test did not pass) and there were no problems after that with any CPU. Overclocking test had no problems at all.
Next Am5x86-133:
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133MHz 4..no 5x86 sure sounds impressive, doesn't it? Too bad the AMD DX4 core is inferior to Intel, at least if speed is the main thing and by naming it to 5x86 is surely must be.
At worst the extra 33 MHz is needed to match 100 MHz Intel DX4 WB. But at its best Doom was 17% faster and Doom is a real world benchmark. So it is nowhere near as fast as 133 MHz suggest it to be. But it was never really slower than Intel DX4-100 WB but uses a lot less power so it produces a lot less heat. This is the best feature of Am5x86. There is very little speed difference between 120 MHz and 133 MHz. What you lose from CPU clock you gain from memory clock. But with L1 disabled, the 120 MHz is the faster one. So overall, Am5x86 is faster at 120 MHz than at 133 MHz. And as a bonus, it then identifies "correctly" as Am486 instead of Am5x86. I really hate to call this 5x86.
But this CPU has issue with disabling L1 cache. I observed this with maybe at three different speeds so this is not a one time glitch. Disabled L1 cache seems to partially enable randomly. I had to retest several L1D tests because the results were 50-80% too high and this very little L1 cache did not get enabled at same point every time.
For example when overclocked to 160 MHz, I started from L1D test the usual way. First Quake or Doom, then the other of those two. They are so sloooooww so I'll just do something else while the tests are running. Next Doom with min graphics, then PC Player tests (first SVGA), then 3DBench and very last Topbench. It was 3Dbench where the false readings started. But on other tested speeds it was at some other test at much earlier.
And the 50-80% is too much of an increase in speed that there will be OPL related issues on the most problematic games. No other CPU had this kind of issue and it happened many times. Too bad, Am5x86-133 would have been great CPU otherwise.
Last Cyrix 5x86:
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Really good CPU. Fast with little less power consumption than Intel CPUs. Overclocked really well to 120 MHz: completely stable with 3,45V and power consumption increased only little. PC Player benchmarks favors Intel quite a lot for some reason. Other than that, the Cyrix is faster and with some control register tuning it gets even faster. But still not fast enough to beat Intel at PC Player benchmarks. Overall, Cyrix is the fastest 100 MHz CPU and fastest 120 MHz CPU. 100 MHz is not enough to outperform Am5x86-133 but 120 MHz is enough for that, even without tuning.
But there is the issue with 1x multiplier. I'm unfair here because it is a feature that the other CPUs do not have at all. And even the Cyrix does not really need it since same L1D trick slows down just like with the other CPUs (expect Intel WT that needed de-turbo as well). So first I needed to disable the certain BIOS setup feature to be able to use 1x multiplier with L2 cache enabled. Even then it did not work with UMB driver but it did work if EMM386 was used to create UMBs.
I think I have explanation for that. Cyrix 6x86 does have a feature in its control registers to write protect 640k-1MB memory area. That is where the UMB area is. Cyrix 5x86 does not have that bit in its registers but that write protect would explain perfectly the issues I'm having. With EMM386 those UMBs are not actually located in the 640k-1MB area but remapped to somewhere else. The physical UMB area is reserved by chipset unless real mode UMB driver is used.
So what I think is that this S1R3 CPU I'm having somehow manages to enable the write protection while entering 1x multiplier mode and when switching back to 3x multiplier mode, the write protection also disables. So a CPU bug. That is why I would be very interested to try the S0R5 CPU (that should be later even though it is Stepping 0).
Since I would most likely use the 1x multiplier only when using EMM386 as well (so I can use MIDIto) this is not a big issue.
I don't like 486 systems with PCI but I understand the benefits: much easier to find a good VGA card, integrated I/O so BIOS can enable PIO4 and one big card less and so on. If I would build a PCI 486 system, I'd definitely go with Cyrix 5x86. It is a very good CPU and would be a perfect match for a late 486 PCI motherboard.
But what do I choose for this system? I'll think about it some more. It is either Intel WB or Cyrix. Am5x86 doesn't belong here, not with its L1D issue. I do admit that lower power consumption would be nice.
