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Multiplier control for 486

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First post, by chublord

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How is the multiplier set on DX4's and Am5x86's?

My system does the following

DX4-100 @ 33 MHz FSB = 100 MHz (3x)
DX4-100 @ 40 MHz FSB = 80 MHz (2x)
DX4-100 @ 50 MHz FSB = 100 MHz (2x)

I thought the processor had a default multiplier? Somehow my chipset is overriding it? What would happen if I tried a Am5x86-133 @ 40 or 50 MHz FSB?

It's a IBM Valuepoint system, Opti 802 chipset. There aren't any explicit multiplier jumpers on the motherboard, just a FSB setting.

IBM Valuepoint 486 DX4-100, Opti 802G, 50 MHz FSB, Voodoo1+S3 864, Quantum Fireball EX 4.0 GB, Seagate Medalist 1.6 GB, 128 MB FPM, 256k L2

Reply 1 of 4, by Horun

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Picture of the exact board would help. Yes the chipset + BIOS can override default CPU multpliers or down clock the cpu when it thinks it is set wrong.

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. Stuff: https://archive.org/details/@horun

Reply 2 of 4, by Anonymous Coward

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486DX does not have multipliers. The 5V DX2 chips have a 2X multiplier that cannot be changed. 3.3V DX2s and DX4s have an extra pin that allows you to select between 2X and 3X using a motherboard jumper. am5x86 and later am486DX4 have the same multiplier pin but you can only choose between 3X and 4X multipliers. Then you have some weird chips like the 486SXL(2) start up in 1X mode and use software to toggle a 2X mode (but these are extremely rare in 168 pin versions). Finally, you have the Cyrix 5x86. These were available in two versions. Initially as a 2X/3X selectable model and later as 3X/4X. The Cyrix 5x86 is kind of special though in that with software you can drop them down to 1X mode. Perhaps some of the Cyrix based 486s can do this too? I'm not certain.

So the answer to your question is that it depends on what CPU you have, and there aren't any flexible tools like setmul that let you set whatever multiplier you want.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 3 of 4, by chublord

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Thanks for the info! So it sounds like if I used a Am586 @ 50 Mhz FSB it would use a 3x multiplier, which is what I wanted.

This is the motherboard, it's not a standard board (IBM system).

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IBM Valuepoint 486 DX4-100, Opti 802G, 50 MHz FSB, Voodoo1+S3 864, Quantum Fireball EX 4.0 GB, Seagate Medalist 1.6 GB, 128 MB FPM, 256k L2

Reply 4 of 4, by Disruptor

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chublord wrote on 2020-07-19, 18:20:

Thanks for the info! So it sounds like if I used a Am586 @ 50 Mhz FSB it would use a 3x multiplier, which is what I wanted.
...

Yes, but ensure the multiplier jumper is set to x3 at a lower FSB (eg. at 33 MHz).
When you have it in wrong x4 at 50 MHz...

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