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First post, by aries-mu

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Hello folks,

I was wondering: I never saw 80387 40MHz FPUs around.
So, if one has an AMD 386 DX 40 MHz and wants to add a FPU, I guess should add an Intel 80387 33 MHz. Wouldn't that cause the CPU to downclock to 33 MHz?
Or, would the CPU force overclock the FPU to 40MHz? If so, will this cause instability or, worse, burn the FPU?

Any other suggestion to have a FPU working at 40 MHz reliably and safely?

Thanks.

They said therefore to him: Who are you?
Jesus said to them: The beginning, who also speak unto you

Reply 1 of 11, by jasa1063

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aries-mu wrote on 2021-11-29, 22:40:
Hello folks, […]
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Hello folks,

I was wondering: I never saw 80387 40MHz FPUs around.
So, if one has an AMD 386 DX 40 MHz and wants to add a FPU, I guess should add an Intel 80387 33 MHz. Wouldn't that cause the CPU to downclock to 33 MHz?
Or, would the CPU force overclock the FPU to 40MHz? If so, will this cause instability or, worse, burn the FPU?

Any other suggestion to have a FPU working at 40 MHz reliably and safely?

Thanks.

There are 40MHz 387 FPUs from Cyrix, IIT & ULSI. I have 2 from Cyrix in my two 386/40 builds. Here is a great article that goes into detail on all of these:

https://dougx.net/gaming/coproc.html

Reply 2 of 11, by aries-mu

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jasa1063 wrote on 2021-11-29, 22:56:
aries-mu wrote on 2021-11-29, 22:40:
Hello folks, […]
Show full quote

Hello folks,

I was wondering: I never saw 80387 40MHz FPUs around.
So, if one has an AMD 386 DX 40 MHz and wants to add a FPU, I guess should add an Intel 80387 33 MHz. Wouldn't that cause the CPU to downclock to 33 MHz?
Or, would the CPU force overclock the FPU to 40MHz? If so, will this cause instability or, worse, burn the FPU?

Any other suggestion to have a FPU working at 40 MHz reliably and safely?

Thanks.

There are 40MHz 387 FPUs from Cyrix, IIT & ULSI. I have 2 from Cyrix in my two 386/40 builds. Here is a great article that goes into detail on all of these:

https://dougx.net/gaming/coproc.html

Wow! Thank you!

They said therefore to him: Who are you?
Jesus said to them: The beginning, who also speak unto you

Reply 4 of 11, by aries-mu

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kixs wrote on 2021-11-29, 23:52:

Intel never did a 386 @ 40Mhz so that's why it never did 387@40Mhz. Although it overclocks just fine.

Interesting!

They said therefore to him: Who are you?
Jesus said to them: The beginning, who also speak unto you

Reply 6 of 11, by bloodem

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I always go for the IIT FPUs (those in aluminum package).
Even though they're definitely not the fastest (but not the slowest either), they simply look awesome to me (see attachment)

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2 x PGA132 / 5 x Socket 3 / 9 x Socket 7 / 12 x SS7 / 1 x Socket 8 / 14 x Slot 1 / 5 x Slot A
5 x Socket 370 / 8 x Socket A / 2 x Socket 478 / 2 x Socket 754 / 3 x Socket 939 / 7 x LGA775 / 1 x LGA1155
Current rig: Ryzen 5 3600X
Backup rig: Core i7 7700k

Reply 7 of 11, by dionb

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aries-mu wrote on 2021-11-29, 22:40:
Hello folks, […]
Show full quote

Hello folks,

I was wondering: I never saw 80387 40MHz FPUs around.
So, if one has an AMD 386 DX 40 MHz and wants to add a FPU, I guess should add an Intel 80387 33 MHz. Wouldn't that cause the CPU to downclock to 33 MHz?
Or, would the CPU force overclock the FPU to 40MHz? If so, will this cause instability or, worse, burn the FPU?

Any other suggestion to have a FPU working at 40 MHz reliably and safely?

Thanks.

You seem to have some odd ideas about clocking...

Neither CPU nor FPU "forces" anything, or decides anything related to clocking themselves. They just run on the clock they are given by the system - or fail to do so. Their rated speed is nothing more than an indication from te vendor how fast they can/should be run. See it like speed limits on a road. The road doens't determine how fast you drive, the driver does. However if you decide to try to go well over the limit you might crash. Of course, sometimes roads have posted limits well below what is technically possible. That's your overclocking headroom.

Reply 8 of 11, by aries-mu

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nice stuff, thanks guys. Love the analogy Dionb.

And yes, sometimes the speed limits suck insanely, see Toronto.

They said therefore to him: Who are you?
Jesus said to them: The beginning, who also speak unto you

Reply 9 of 11, by Horun

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jasa1063 wrote on 2021-11-29, 22:56:

There are 40MHz 387 FPUs from Cyrix, IIT & ULSI. I have 2 from Cyrix in my two 386/40 builds. Here is a great article that goes into detail on all of these:

https://dougx.net/gaming/coproc.html

Great article, thanks !

Anyone buy one of the IIT from the china source on ebay ? Have a ULSI and a Weitek but am wanting a IIT....

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.

Reply 10 of 11, by jasa1063

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Horun wrote on 2021-11-30, 23:17:
jasa1063 wrote on 2021-11-29, 22:56:

There are 40MHz 387 FPUs from Cyrix, IIT & ULSI. I have 2 from Cyrix in my two 386/40 builds. Here is a great article that goes into detail on all of these:

https://dougx.net/gaming/coproc.html

Great article, thanks !

Anyone buy one of the IIT from the china source on ebay ? Have a ULSI and a Weitek but am wanting a IIT....

I bought an IIT 387SX/40 from China on eBay. It works just fine and passes all tests using IEEETEST. Here is a link to the eBay store I bought from.

https://www.ebay.com/str/zoescpustore

Reply 11 of 11, by Horun

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jasa1063 wrote on 2021-12-01, 02:49:
Horun wrote on 2021-11-30, 23:17:

Anyone buy one of the IIT from the china source on ebay ? Have a ULSI and a Weitek but am wanting a IIT....

I bought an IIT 387SX/40 from China on eBay. It works just fine and passes all tests using IEEETEST. Here is a link to the eBay store I bought from.

https://www.ebay.com/str/zoescpustore

Thanks ! Grabbed 2 IIT PGA types for my DX boards.

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.