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486 SX 16 & 20 MHz

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First post, by mpe

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Got these two babies.

I was wondering if there is a regular (non-OEM) motherboard that can drive these at 16 or 20 MHz. All my 486 motherboards start at 25 MHz.

I was thinking using an older 486 motherboard and replacing the crystal. But hope there might be a better way...

AWbSE4G.jpg

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Reply 1 of 91, by Nvm1

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I have a compaq system with a 16mhz 486. My understanding is that the 16mhz version only was used in OEM systems. The 20mhz version is usable on early isa only boards. And because those where replaced with better boards soon there arent many left.

Reply 2 of 91, by jaZz_KCS

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wow, a genuine 486-SX16. The only way I was able to reach those low speeds with an 486-SX was using a 486-SX33, which is rather common and halfin the clock speed/setting it to low in the BIOS.

Reply 3 of 91, by BinaryDemon

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Does a 386DX-40 beat a 486SX-16? But lose to the 486SX-20? You don’t see many benchmarks for those systems.

Check out DOSBox Distro:

https://sites.google.com/site/dosboxdistro/ [*]

a lightweight Linux distro (tinycore) which boots off a usb flash drive and goes straight to DOSBox.

Make your dos retrogaming experience portable!

Reply 5 of 91, by mpe

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BinaryDemon wrote:

Does a 386DX-40 beat a 486SX-16? But lose to the 486SX-20? You don’t see many benchmarks for those systems.

This is actually what I would like to experimentally prove.

I expect that the 486SX 16 MHz should beat 386DX-33 and the 486-SX 20MHz shouldn't have any problem beating 386DX-40.

Most 386 DX boards would be disadvantaged by only 64k of slow cache, no vl-bus, etc.

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Reply 6 of 91, by derSammler

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AlessandroB wrote:

386 and 486 are not far clock to clock, probably 386DX40 is much faster.

No, they are quite far from each other if you compare the CPU only. A 486 is much faster. A 386 can only come close to a 486 if external cache is present. This, however, is no longer a fair comparison, as the external cache is provided by the mainboard with a 386, not by the CPU (unlike with the 486).

Reply 7 of 91, by bakemono

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I used to have a 486SX-16. It was pretty close to a 386DX-25 with cache.

IIRC, the PX486P3 board had undocumented jumper settings for 16 and 20MHz. I'm sure some other boards also have such undocumented settings.

Reply 8 of 91, by Scali

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BinaryDemon wrote:

Does a 386DX-40 beat a 486SX-16? But lose to the 486SX-20? You don’t see many benchmarks for those systems.

From what I recall, the 486SX-25 is closest to the 386DX-40.
The 20 and 16 would be slower, a 33 would be faster.

http://scalibq.wordpress.com/just-keeping-it- … ro-programming/

Reply 9 of 91, by Horun

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mpe wrote:

Got these two babies.
I was wondering if there is a regular (non-OEM) motherboard that can drive these at 16 or 20 MHz. All my 486 motherboards start at 25 MHz.
I was thinking using an older 486 motherboard and replacing the crystal. But hope there might be a better way...]

Really nice ! slowest 486 in my collection is a 486DX-25, have read about the SX-16 in some older mag's but have never seen one until now.

Hate posting a reply and have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. 🤣 Second computer a 286 12Mhz with real IDE drive ! After that came 386, 486, Pentium, P.Pro and everything after....

Reply 11 of 91, by Cyberdyne

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They usually run totally good in 25mhz and even 33mhz.

I am aroused about any X86 motherboard that has full functional ISA slot. I think i have problem. Not really into that original (Turbo) XT,286,386 and CGA/EGA stuff. So just a DOS nut.

Reply 12 of 91, by H3nrik V!

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Yeah, the 16 MHz is pretty rare, as it was only sold to OEMs .. I own a 20 MHz, that a colleague one day just came by and gave me 😀

Please use the "quote" option if asking questions to what I write - it will really up the chances of me noticing 😀

Reply 13 of 91, by mpe

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bakemono wrote:

IIRC, the PX486P3 board had undocumented jumper settings for 16 and 20MHz. I'm sure some other boards also have such undocumented settings.

Actually this was a great idea. Thanks!

I noticed that on one of my motherboards there is UM9515-1 clock generator. Removing all clock jumpers switched it to 20 MHz.

So the 20 MHz i covered, now I need to find a board with 16 MHz clock.

The 486SX-20 MHz seems to be a bit faster than my 386DX-40. Like 9.5 v 8.3fps in Doom. But thats with different videocard, more cache in 486, ... Many synthetic benchmarks show almost identical results.

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Reply 14 of 91, by Caluser2000

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Scali wrote:
BinaryDemon wrote:

Does a 386DX-40 beat a 486SX-16? But lose to the 486SX-20? You don’t see many benchmarks for those systems.

From what I recall, the 486SX-25 is closest to the 386DX-40.
The 20 and 16 would be slower, a 33 would be faster.

That is what I got from reading some old USENET posts. Seems the earlier 486SXs had the FPUs disabled but the later SX25s up had the fpu removed from the die completely.

Last edited by Caluser2000 on 2019-10-28, 05:45. Edited 1 time in total.

There's a glitch in the matrix.

Reply 15 of 91, by H3nrik V!

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Caluser2000 wrote:
Scali wrote:
BinaryDemon wrote:

Does a 386DX-40 beat a 486SX-16? But lose to the 486SX-20? You don’t see many benchmarks for those systems.

From what I recall, the 486SX-25 is closest to the 386DX-40.
The 20 and 16 would be slower, a 33 would be faster.

That i swhat I got from reading some old USENET posts. Seems the earlier 486SXs had the FPUs disabled but the later SX25s up had the fpu removed from the die completely.

Probably first many SX'es were DX'es with errors in the FPU. Then had FPU disabled and sold as SX. Later on, DX yield was probably high enough, then SX were produced without the actual FPU ..

Please use the "quote" option if asking questions to what I write - it will really up the chances of me noticing 😀

Reply 17 of 91, by Scali

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H3nrik V! wrote:

Probably first many SX'es were DX'es with errors in the FPU. Then had FPU disabled and sold as SX. Later on, DX yield was probably high enough, then SX were produced without the actual FPU ..

Yup, the first part was getting rid of the broken DX chips.
But probably because of the success of the AMD 386DX40, Intel continued with the 486SX. There was still a good market for this kind of budget CPU without FPU.

http://scalibq.wordpress.com/just-keeping-it- … ro-programming/

Reply 18 of 91, by mpe

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Isn't great that in 2019 people are still being fascinated that some 486SX's may contain a real FPU 😀

Personally I think that although there is enough evidence that the FPU is physically there in early SX units (and absent in some later chips) I am not so sure it was the way of dealing with broken DX chips.

In 1991 I doubt they had problems with 486DX yields. A simple connection removal by laser wouldn't change CPU identification (and no microcode update capability on chips before P6). I believe in 1991 it was simply a good time to have a cheaper 486 product.

A great article about the topic:
http://www.os2museum.com/wp/lies-damn-lies-and-wikipedia/

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Reply 19 of 91, by Scali

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mpe wrote:

Personally I think that although there is enough evidence that the FPU is physically there in early SX units (and absent in some later chips) I am not so sure it was the way of dealing with broken DX chips.
...
I believe in 1991 it was simply a good time to have a cheaper 486 product.

I think these two statements contradict eachother somewhat.
Early 486SX CPUs actually did contain the FPU, so they were technically the same die as a 486DX. Which means they weren't cheaper to produce.
Given that they'd also require new motherboards, with an FPU socket, where is the cost saving?

Perhaps the truth is somewhere in the middle:
Intel encountered yield issues with early steppings of 486, mainly in the FPU.
So they introduced a new stepping (ergo CPU identification changed), where they could laser off the FPU if it failed validation.

Eventually, the market changed: more demand for 486SX, but yields also improved, so less 'lasered off' chips available.
So Intel introduced another version of the 486SX, with the FPU physically removed, to reduce cost, and meet market demand for the 486SX.

http://scalibq.wordpress.com/just-keeping-it- … ro-programming/