It depends on the bottleneck your system has. With higher FSB, you get everything on your motherboard (cache, ram, VLB bus) working faster. With multiplier, it's only going to be the cpu itself.
At the same time, it's impossible to find VLB videocards, ram, cache that would work at 100MHz+ speed, while 133MHz 486 CPUs are quite common. (I run one at 3x50)

Both multiplier and FSB speeds are controlled by the motherboard. The CPU just sets limits, what multiplier options are available.

GigAHerZ wrote on 2023-09-29, 08:15:

It depends on the bottleneck your system has. With higher FSB, you get everything on your motherboard (cache, ram, VLB bus) working faster. With multiplier, it's only going to be the cpu itself.
At the same time, it's impossible to find VLB videocards, ram, cache that would work at 100MHz+ speed, while 133MHz 486 CPUs are quite common. (I run one at 3x50)

Both multiplier and FSB speeds are controlled by the motherboard. The CPU just sets limits, what multiplier options are available.

Got it, thanks!

But that's fantastic! Does that mean that, provided the mobo has such options, one could get a VLB 486 DX4 100 and set it up in such way?

Bus: 50 MHz
CPU: bus x 2 = 50 x 2 = 100 MHz

I mean, if the CPU can go x3, it should surely be able to go x2

And, as for the frequency, if the CPU can handle 99 MHz on a 33x3 setup, for sure it should be able to handle 100 MHz on a 50x2 setup, that's just 1 MHz higher than its natural state, isn't it?

Correct.
You just need to be aware that with 50MHz FSB, your ram, cache, VLB would be able to keep up with that speed. (Some added waitstates may help)
Also make sure that your ISA bus doesn't go too much off the spec. Usually 10-12MHz are still fine with most cards.

GigAHerZ wrote on 2023-09-29, 08:53:

Correct.
You just need to be aware that with 50MHz FSB, your ram, cache, VLB would be able to keep up with that speed. (Some added waitstates may help)
Also make sure that your ISA bus doesn't go too much off the spec. Usually 10-12MHz are still fine with most cards.

Yes sure. About the cards and cache. It would be useful to create a thread, actually I'm gonna create it now....

It's here:
Cache, RAM, graphic cards, and controllers that can handle higher FSB speeds

About the ISA problem: I wasn't aware of that! 😳 I always thought that ISA worked well and stably at 16 MHz and so did all the 16 Bit ISA cards by default!!! 😳

And, how can one regulate the ISA bus speed, based on the VLB speed? Is there gonna be other jumpers? A multiplier??? (like, VLB / 2 or / 3 ???)

aries-mu wrote on 2023-09-29, 09:37:

About the ISA problem: I wasn't aware of that! 😳 I always thought that ISA worked well and stably at 16 MHz and so did all the 16 Bit ISA cards by default!!! 😳

And, how can one regulate the ISA bus speed, based on the VLB speed? Is there gonna be other jumpers? A multiplier??? (like, VLB / 2 or / 3 ???)

In bios usually there is a setting to set ISA = FSB/4, FSB/5, etc. FSB is usually CLK or CLK2 or something like that.

Many cards can go further than 12MHz, sure. But i consider it an experimentation area. For a stable system where i don't want compatibility problems, i would always go for under 12MHz ISA.

GigAHerZ wrote on 2023-09-29, 10:36:

In bios usually there is a setting to set ISA = FSB/4, FSB/5, etc. FSB is usually CLK or CLK2 or something like that.

Many cards can go further than 12MHz, sure. But i consider it an experimentation area. For a stable system where i don't want compatibility problems, i would always go for under 12MHz ISA.

Oh wow! Useful info, thanks!

When you cant decide...

DX 50 or DX2 50 ... Thats the question 😁

DerBaum wrote on 2023-09-29, 10:58:

When you cant decide...
2023-09-29 12.39.13.jpg
DX 50 or DX2 50 ... Thats the question 😁

Heheheheh! I have the perfect thread for you about this, I remember I created it a long time ago. Here, enjoy:

The good old and glorious 486 DX 50 - and - rant against 25 MHz buses in the Pentium era

Plus, a similar one that I just created:
DX2 set at 33/66 VS same CPU but set at 50/50 - opinions?

Intel's 486DX4-100 in sSpec SK050 may be qualified to run at 2x50 according to https://www.cpu-world.com/sspec/SK/SK050.html

But I would think that the multiplier was set by the jumpers on the motherboard. But whether or not an x2 multiplier is "always" supported by a DX4, I'm not sure ...

H3nrik V! wrote on 2023-09-29, 12:19:

Intel's 486DX4-100 in sSpec SK050 may be qualified to run at 2x50 according to https://www.cpu-world.com/sspec/SK/SK050.html

Apparently the SX877 as well ..? https://www.cpu-world.com/sspec/SX/SX877.html

H3nrik V! wrote on 2023-09-29, 12:19:

But I would think that the multiplier was set by the jumpers on the motherboard. But whether or not an x2 multiplier is "always" supported by a DX4, I'm not sure ...

Yes it's set by motherboard, but finding where exactly can be a challenge, as many 486 motherboards obfuscate actual jumper functions and just list settings per CPU.

H3nrik V! wrote on 2023-09-29, 12:20:

H3nrik V! wrote on 2023-09-29, 12:19:

Intel's 486DX4-100 in sSpec SK050 may be qualified to run at 2x50 according to https://www.cpu-world.com/sspec/SK/SK050.html

Apparently the SX877 as well ..? https://www.cpu-world.com/sspec/SX/SX877.html

oh good! Interesting, thank you!

What location of multipler pin setting on the Intel 486DX4? So I can document more jumpers on a unknown SBC.

Cheers,

I know they're not all supposed to use the same pins, between Intel AMD and Cyrix, think it's AMD and Intel the same Cyrix different, but it's foggy in the old memory. Then also reputed to exist are DX4 culls from AMD and Cyrix that are sold as DX2 80 but will also run on a 3x multi... but those will be 3.3V parts or 3.5 or 3.45 or 3ish not 5V anyway.

aries-mu wrote on 2023-09-29, 08:11:

Guys, I was wondering... […]
Show full quote

Guys, I was wondering...

Since VLB mobos can be clocked @40 and even 50 MHz on the bus... I was wondering if it would be possible, for example, to grab an Intel DX4 100 MHz, and, instead of setting up the bus @ 33 MHz, setting it up @ 50 MHz and modify the multiplier to x2 instead of x3. Is something like this possible? What dictates the CPU clock multiplier? The mobo jumpers or the CPU itself? Cause, if the latter, I doubt we can put our hands on it...

Well, and even if we cannot, ASSUMING we could, I wonder what would perform better:

• Intel 486 DX4 100 multiplied at 50 MHz x 2
• AMD 486 DX4 120 with its natural 40 MHz x 3 setting

Everything else the same (same L1 and L2 cache, size and type, same RAM size and type, same 0 wait states on the VLB cards, same SVGA and drive controller... same everything else)

Thanks

CPUs manufactured then were binned/graded based on in-factory testing. Some chips can handle 'incorrect' clocking better than others, depending on multiple factors that were in play when it was manufactured (Billet of silicon it was produced from, what batch of UV sensative etching/resist was used, etc...)

In a more direct answer to your question, there were some "Oddball" 486 chips that had a 1x multiplier. There *WERE* DX-50 chips, for instance. (NOT a DX/2-50, no, I mean a straight up DX-50)
https://dfarq.homeip.net/486dx-50-versus-dx2/

These chips were either "Blazing fast!" or "Cause so much horrible headache that they are useless!!" depending on where and how you put them into service. VLB motherboards for instance, were NOTORIOUS for **NOT** playing nice with these chips, as 50mhz was pushing the extreme edge of what the VLB architecture could accomplish. VLB is designed with a nominal bus speed of 33mhz, and 50mhz is some 50% higher than that. Not all cards could handle being driven that fast, not all implementations of the VLB could tolerate being driven at that speed reliably, and then there were other factors that might come into play (like shitty capacitors with poor noise suppression, crappy wiring in people's houses, people not using grounded outlets, et al.)

The actual answer to your question, is "If the CPU's bus speed stays at or near the nominal 33mhz, there is no real need for a clock multiplier by the chip, except that you will have crummier performance for in-processor applications-- at least as far as IO Bus technologies of the era were concerned." Clock multipliers allowed the chip to do things quickly internally (EG, multiple processor cycles could be done per IO cycle, and software written to take advantage of this fact, could do amazing things) while "Doing the best they could" with on-board discrete components, and the delays and other issues that circuit design and component selection from the era imposed.

I had a 486 that was ISA only; The DX50 I popped in it worked GREAT.
I later got a board that could do VLB, and realized quickly that my DX50 caused me no end of grief.

I sorely missed being able to play MP3s on my 486 (The DX50 could do raw RAM IO fast enough to decode MP3, which was something normally reserved for Pentium species chips) but being able to have better video performance and other gains elsewhere were a fair trade, when "Demoting" to a DX/2-50.

I suppose on a rare PCI equipped 486, the DX50 would be a fine choice. (though that assumes bus-mastering is present on that ancient PCI chipset)

aries-mu wrote on 2023-09-29, 08:11:

Guys, I was wondering... […]
Show full quote

Guys, I was wondering...

Since VLB mobos can be clocked @40 and even 50 MHz on the bus... I was wondering if it would be possible, for example, to grab an Intel DX4 100 MHz, and, instead of setting up the bus @ 33 MHz, setting it up @ 50 MHz and modify the multiplier to x2 instead of x3. Is something like this possible? What dictates the CPU clock multiplier? The mobo jumpers or the CPU itself? Cause, if the latter, I doubt we can put our hands on it...

Well, and even if we cannot, ASSUMING we could, I wonder what would perform better:

• Intel 486 DX4 100 multiplied at 50 MHz x 2
• AMD 486 DX4 120 with its natural 40 MHz x 3 setting

Everything else the same (same L1 and L2 cache, size and type, same RAM size and type, same 0 wait states on the VLB cards, same SVGA and drive controller... same everything else)

Thanks

I saw you opened tons of threads about 486, which is my main retro interest topic.
My opinion about 50MHz FSB:
Forget it.
Unless you have special SRAMs, 50MHz means you have to run the L2 cache with 3-2-3. That means read 3-2-2-2 and write 3 clock cycles.
With 33MHz you always can use 2-1-2 timing. So in that case L2 access for 33MHz is FASTER than L2 access at 50MHz. 60-30-60 ns for 33MHz, 60-40-60 nanoseconds for 50MHz.
Additionally with VLB but also PCI at 50MHz one can have addtional problems.

So I think for the Intel DX4 33*3 setting performance is very close to 50*2 setting. Assuming that one gets the system stable at 50MHz.
But a different story is 40MHz FSB. With many boards, you are able to achieve 2-1-2 with 40MHz. That is really fast.

So about the Am5x86, when you are a bit lucky you can ran it at 40*4 = 160MHz, when this works you can of course also use 50*3 (despite you may have more stability problems).
A lot of people here on Vogons claim, that in some benchmarks the 150MHz settings beats the 160MHz setting.
I cannot confirm that, for me 160MHz was better in every benchmark like doom, quake, Wolfenstein3D and so on.

A different story is 60MHz and 66MHz FSB on a 486. But only late PCI boards can achieve that setting, and you need either an AMD 486-120 (to run it 60*2). Or for a Am5x86 that means either 180 or 200MHz, which is hard to achieve. So I have no experience with 60, 66MHz FSB.

Classical DX2 is multiplier locked or at least there are no obvious ways to tackle multiplier registers, because that CPU was designed as a direct replacement for DX33 parts. Rebadged DX4 can switch between 2X and 3X multipliers.

wierd_w wrote on 2023-09-30, 07:25:

CPUs manufactured then were binned/graded based on in-factory testing. Some chips can handle 'incorrect' clocking better than o […]
Show full quote

CPUs manufactured then were binned/graded based on in-factory testing. Some chips can handle 'incorrect' clocking better than others, depending on multiple factors that were in play when it was manufactured (Billet of silicon it was produced from, what batch of UV sensative etching/resist was used, etc...)

In a more direct answer to your question, there were some "Oddball" 486 chips that had a 1x multiplier. There *WERE* DX-50 chips, for instance. (NOT a DX/2-50, no, I mean a straight up DX-50)
https://dfarq.homeip.net/486dx-50-versus-dx2/

These chips were either "Blazing fast!" or "Cause so much horrible headache that they are useless!!" depending on where and how you put them into service. VLB motherboards for instance, were NOTORIOUS for **NOT** playing nice with these chips, as 50mhz was pushing the extreme edge of what the VLB architecture could accomplish. VLB is designed with a nominal bus speed of 33mhz, and 50mhz is some 50% higher than that. Not all cards could handle being driven that fast, not all implementations of the VLB could tolerate being driven at that speed reliably, and then there were other factors that might come into play (like shitty capacitors with poor noise suppression, crappy wiring in people's houses, people not using grounded outlets, et al.)

The actual answer to your question, is "If the CPU's bus speed stays at or near the nominal 33mhz, there is no real need for a clock multiplier by the chip, except that you will have crummier performance for in-processor applications-- at least as far as IO Bus technologies of the era were concerned." Clock multipliers allowed the chip to do things quickly internally (EG, multiple processor cycles could be done per IO cycle, and software written to take advantage of this fact, could do amazing things) while "Doing the best they could" with on-board discrete components, and the delays and other issues that circuit design and component selection from the era imposed.

I had a 486 that was ISA only; The DX50 I popped in it worked GREAT.
I later got a board that could do VLB, and realized quickly that my DX50 caused me no end of grief.

I sorely missed being able to play MP3s on my 486 (The DX50 could do raw RAM IO fast enough to decode MP3, which was something normally reserved for Pentium species chips) but being able to have better video performance and other gains elsewhere were a fair trade, when "Demoting" to a DX/2-50.

I suppose on a rare PCI equipped 486, the DX50 would be a fine choice. (though that assumes bus-mastering is present on that ancient PCI chipset)

Wow interesting, thanks!
Well, if you're so fond of the DX50 and if it performs so well on ISA-only mobos (it must be thanks to its 50 MHz FSB), then how about this: Get an AMD DX2-80 if you wanna try VLB!
I mean, the FSB runs at 40, which still is not bad at all, closer to 50 than a 25 or 33 Mhz bus. Plus you get a whopping 80 MHz CPU, well 30 MHz above your DX50. At the same time, the VLB should be able to handle 40 MHz smoothly, and also, if you get a VLB controller card and a VLB VGA card that claim to be able to handle 50 MHz at 0 ws, they should work well together at 40 MHz 0 ws. I suspect those MP3s of yours would fly on such a system (perhaps with 60 ns FPM RAM modules and the best cache you can get). Plus, having VLB controller and vga, the other tasks would also fly.
How about that?

Why demoting to a DX2 50? If you were so struck by the speed of the DX50, why going 25 MHz on the FSB? You could have gotten a DX2 66, faster CPU and faster FSB than the DX2 50.

Very curious: how would the DX50 be a fine choice on a PCI mobo, since PCI is stuck at 33 MHz and cannot be increased??? And what would "bus-mastering" have anything to do with helping with that?

Thanks!

CoffeeOne wrote on 2023-09-30, 09:42:

I saw you opened tons of threads about 486, which is my main retro interest topic. My opinion about 50MHz FSB: Forget it. Unless […]
Show full quote

I saw you opened tons of threads about 486, which is my main retro interest topic.
My opinion about 50MHz FSB:
Forget it.
Unless you have special SRAMs, 50MHz means you have to run the L2 cache with 3-2-3. That means read 3-2-2-2 and write 3 clock cycles.
With 33MHz you always can use 2-1-2 timing. So in that case L2 access for 33MHz is FASTER than L2 access at 50MHz. 60-30-60 ns for 33MHz, 60-40-60 nanoseconds for 50MHz.
Additionally with VLB but also PCI at 50MHz one can have addtional problems.

So I think for the Intel DX4 33*3 setting performance is very close to 50*2 setting. Assuming that one gets the system stable at 50MHz.
But a different story is 40MHz FSB. With many boards, you are able to achieve 2-1-2 with 40MHz. That is really fast.

So about the Am5x86, when you are a bit lucky you can ran it at 40*4 = 160MHz, when this works you can of course also use 50*3 (despite you may have more stability problems).
A lot of people here on Vogons claim, that in some benchmarks the 150MHz settings beats the 160MHz setting.
I cannot confirm that, for me 160MHz was better in every benchmark like doom, quake, Wolfenstein3D and so on.

A different story is 60MHz and 66MHz FSB on a 486. But only late PCI boards can achieve that setting, and you need either an AMD 486-120 (to run it 60*2). Or for a Am5x86 that means either 180 or 200MHz, which is hard to achieve. So I have no experience with 60, 66MHz FSB.

Wait what? Can you run 486 PCI motherboards' FSB at 60 or 66 MHz?? WOW! Can you please mention some examples?
But wasn't PCI inevitably stuck at 33 MHz and not increasable at all? What about PCI cards? Even if you manage to run the PCI bus on the motherboard at 60 or 66 MHz, how can controller and vga cards designed to run at 33 MHz even work? I'm confused...
Unless you're talking about those motherboards with 66 MHz 64 Bit PCI slots, but I don't think so, because you mentioned 486 motherboards. Those, on the contrary, were later Pentium motherboards... And still, good luck finding a 64 bit 66 MHz PCI vga card (hdd controllers maybe...)

Can you please tell me what motherboards can handle well 2-1-2 cache at 40 MHz?

Also, I see many times wait states mentioned on this forum, and you seem to have made some calculations when you extracted 60-30-60 ns and similar out of the wait states numbers. Are there formulas? Is there a tutorial or something? I'd like to be able to understand the meaning of these things.
Also, how would one change those wait states you mentioned? In the BIOS setup or physically via jumpers?

Thanks a lot!