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First post, by bergqvistjl

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I have a Pentium III coppermine with a stock clock of 1Ghz at 133Mhz FSB. Is it safe to underclock this? I'd like to scale it right down to something around 233Mhz, but I don't know if that's possible. My mobo has options for the following settings:

FSB: 66, 100, 133 & 150Mhz
Ratio: 2.0 up to 8.0 plus 0.5 steps.

So if I wanted 266 Mhz, i'd keep the FSB to 133 and lower the ratio down to 2? Would that do it? Or would I end up inadvertently frying the CPU?

Reply 1 of 6, by retardware

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The only problem with underclocking is with dynamic processors (like most Intel processors are) is that the clock gets too low so that the internal processor memories refresh (i.e. registers, cache, etc) cycles get too long, so that the data stored there gets lost.

Just guessing from the things that we know about dynamic memories I suppose that this critical minimum clock rate will be about maybe 10MHz. If you want to be sure, just look up the data sheet and see the minimum clock required there.

Reply 2 of 6, by gerwin

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It is totally safe to underclock with the usual ranges of multipliers and FSB (66MHz minimum).
However, only the FSB part can actually be underclocked, since intel locked the Multiplier to a single value on all retail Pentium 3s. Your CPU will remain at 7.5x.

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Reply 3 of 6, by Koltoroc

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retardware wrote:

The only problem with underclocking is with dynamic processors (like most Intel processors are) is that the clock gets too low so that the internal processor memories refresh (i.e. registers, cache, etc) cycles get too long, so that the data stored there gets lost.

Just guessing from the things that we know about dynamic memories I suppose that this critical minimum clock rate will be about maybe 10MHz. If you want to be sure, just look up the data sheet and see the minimum clock required there.

that is not how it works. On die cache is almost always SRAM and that doesn't need memory refresh. SRAM retains data until it is either overwritten, reset or the power is lost.

Reply 4 of 6, by retardware

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Koltoroc wrote:

that is not how it works. On die cache is almost always SRAM and that doesn't need memory refresh. SRAM retains data until it is either overwritten, reset or the power is lost.

I just named some potential examples for dynamic stuff in processors.
Finding out whether a processor is dynamic, and which parts of it, is usually difficult, as dynamic stuff is inferior to static stuff, which is one reason why manufacturers tend to brag when their processor is static (i.e. clock can be DC), and to be quiet if it is dynamic.

Reply 5 of 6, by dionb

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gerwin wrote:

It is totally safe to underclock with the usual ranges of multipliers and FSB (66MHz minimum).
However, only the FSB part can actually be underclocked, since intel locked the Multiplier to a single value on all retail Pentium 3s. Your CPU will remain at 7.5x.

Yep.

Corollary: if you want a P3-featured CPU running at 266MHz, you need a P3-533EB, which runs at 4x 133MHz by default and will run at 4x 66MHz if you drop the FSB. Back in the day these were popular choices for low-consumption homebrew servers. Given the current state of power management it's pretty pointless - any Core i7 (or i3 or whatever) will use far less at idle than an underclocked P3.

And as for "scaling to around 233MHz" - MHz are a very, very bad measure of performance. They only say something within a CPU architecture, not between them. A P3-533EB running a CPU-limited task will do so almost exactly twice as fast as a P3-533EB@266MHz, but that won't make it just as fast/slow as a P2-266 (although that difference will be minimal), let alone a P266MMX or an Alpha 21164-266.

If you just want to slow down your CPU for DOS gaming, switch off L2 cache for a huge hit and L1 for an even bigger one. dropping FSB can of course also help. If you want more options, consider a Via C3 (which may run on your current motherboard) as they have a massive range of things you can disable to lower performance from a baseline level around that of a P3-500 (in the case of the C3-800)

Reply 6 of 6, by rasz_pl

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retardware wrote:
Koltoroc wrote:

that is not how it works. On die cache is almost always SRAM and that doesn't need memory refresh. SRAM retains data until it is either overwritten, reset or the power is lost.

I just named some potential examples for dynamic stuff in processors.
Finding out whether a processor is dynamic, and which parts of it, is usually difficult, as dynamic stuff is inferior to static stuff, which is one reason why manufacturers tend to brag when their processor is static (i.e. clock can be DC), and to be quiet if it is dynamic.

Its quite easy, just look at manufacturing date. Anything past 1990 is cmos.

bergqvistjl wrote:
I have a Pentium III coppermine FSB: 66, 100, 133 & 150Mhz Ratio: 2.0 up to 8.0 plus 0.5 steps. […]
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I have a Pentium III coppermine
FSB: 66, 100, 133 & 150Mhz
Ratio: 2.0 up to 8.0 plus 0.5 steps.

So if I wanted 266 Mhz, i'd keep the FSB to 133 and lower the ratio down to 2? Would that do it? Or would I end up inadvertently frying the CPU?

you cant change multipliers on p3 coppermines

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