Hey buddy!

That's right up my alley 😀

I take it that machine also has 2nd level cache (Cache that's on the mainboard).

If you are lucky there are BIOS settings to disable L1 and L2 cache.

L1 cache can be disabled via software worst case, but not L2.

So worst case you can only disable L1 cache and then you would have a slow 486DX2.

IF you can also disable L1 Cache you will have a fast 386DX.

Mau1wurf1977 wrote:

Hey buddy! […]
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Hey buddy!

That's right up my alley 😀

I take it that machine also has 2nd level cache (Cache that's on the mainboard).

If you are lucky there are BIOS settings to disable L1 and L2 cache.

L1 cache can be disabled via software worst case, but not L2.

So worst case you can only disable L1 cache and then you would have a slow 486DX2.

IF you can also disable L1 Cache you will have a fast 386DX.

I don't understand how this works. If turning off the cache on a K6-2 550 slows it to 386-25 speed, then the implication here is that if you were to add the same amount of cache to a 386-25, that it would run at K6-2 550 speed and that obviously can't be right.

No, it doesn't work that way.

By deactivating the cache on a Pentium MMX or K6-2, you're adding a significant bottleneck to the CPU. It can process data much faster, but due to the deactivated cache, it stays at the speed of a 386.

Adding more cache to a 386 won't increase the performance by a whole lot. While it's true that additional cache does increase the performance, you won't be able to give the CPU a significant boost, since it's already running at more or less maximum performance. There's not really any bottleneck at the 386 CPU that would be removed by adding more cache.

Remember:
The 386 CPU achieves its performance with active cache and gets the most performance with its 40MHz.
The Pentium233MMX is severely limited by the cache deactivation. It still runs at 233MHz, but it can't use them properly.
Adding cache to a 386 won't magically turn the 40MHz into 233 or even 550 MHz. It still runs at 40MHz and there's no way around that.

It might run slightly faster...maybe like a low end 486. Like the P55C was slightly faster than the P55, not because of the MMX instructions, but because of the increased cache size.

This is what happens when you add L1 cache to a 386 -- quite a big performance boost actually. I'm not sure what adding more cache would do though.

Conversely, turning the L1 cache off on a Cyrix 5x86 drops performance by 90%, turning it into a 386 as you've seen in your chart. I suspect you meant to say L1 cache instead of L2 cache. Turning off the L2 cache on a Pentium 200 won't turn it into a 386, performance-wise.

The speed hit occurs because all instructions need to be sent directly from the system memory (or perhaps L2 motherboard cache) to the on-chip registers, instead of having fast access, on-chip, 16-32KB buffer storage.