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Pentium MMX fixed multiplier?

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First post, by feltel

Posted on 
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I'm about to build a new rig based on an MSI 5128 v1.1 Socket 7 board and an MMX CPU. I currently have an Pentium MMX 200 SL27J which I want to run at 133 MHz to have it passivly cooled. If I set the switches according to the PCB printing I get weird results.

The board has a row of dip switches which set the FSB and the multiplier. See https://retronn.de/imports/hwgal/hw_mainboard … 5128_430hx.html

Setting it to 133 MHz results in an 166 MHz on BIOS boot. So the CPU runs at 2.5 * 66 vs. 2.0 * 66. Setting it to 75 Mhz (i.e. 1.5 * 50) make this CPU run a 125 MHz. The MMX seems to interpret 2.0-settings as 2.5 and 1.5 as 2.5 also. It seems my CPU has a fixed multiplier and can only be downclocked by lowering the FSB. Am I correct?

Dont want to run it at such a low FSB as it should hit performance on a great level.

Does non-fixed MMX exist?

Reply 1 of 1, by Doornkaat

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Non-fixed MMX exist, yes. The first core stepping wasn't fixed but the stepping didn't include the 3.5x multi. The 1.5x multi is not avaliable on any (desktop) Pentium MMX.
The chips had very good yields so Intel decided to include the 3.5x multi in the next stepping and people quickly started overclocking their 166MHz MMX's becuse most ran fine at 233MHz. The first processors of the second stepping appear to still be unlocked.
Then Intel decided to lock their processors' multipliers. This was done by permanently pulling the BF0 signal low via substrate (the CPU PCB) on the 166MHz model and pulling BF1 low via substrate on the 200MHz model.
This means the die does not recognise one of the jumpers on the motherboard anymore. That is why you can only choose two multipliers on locked Pentium MMX CPUs.
The 233MHz models are all unlocked though.

Edit:
In conclusion: To run your MMX@133MHz you either need an unlocked CPU or any 166MHz model.

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