What is Internal vs Internal MHz

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What is Internal vs Internal MHz

Postby GabrielKnight123 » 2018-4-19 @ 10:19

Im looking to getting the fastest CPU for my 486 motherboard and when I found a manual online from here:

http://www.arvutimuuseum.ee/th99/m/E-H/33912.htm

its says "25/33/40/50(internal)/50/66(internal)MHz" for processor speed but on the motherboard it only says "25MHz, 33MHz, 40MHz and 50MHz"

The CPU types the Mobo accepts are:
CX486S(WB)/CX486S(WT)/CX486S2(WB)/CX486S2(WT)/80486SX/ 80487SX/CX486DX(WB)/CX486DX(WT)/CX486DX2(WB)/CX486DX2(WT)/80486DX/80486DX2

I found an i486 DX2 66MHz would this be ok to use?
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Re: What is Internal vs Internal MHz

Postby dionb » 2018-4-19 @ 10:43

Manualspeak is sometimes so lovely ;)

This is about clock doubling (DX2 / 2x multiplier).

Stock 486 bus speeds are 16, 20 (these two 486SX only), 25, 33, 40 and 50MHz, and the original 486 ran at the same speed as its bus. When CPU (and so bus) clocks reached 50MHz, there were two general conclusions:
1) Still more speed was going to be required.
2) at 40 but particularly 50MHz bus speed, things started getting unstable. VLB cards and to lesser degree cache and system RAM had trouble keeping up, giving 50MHz bus systems a reputation for instability.

So, how to get more CPU speed without killing everything on the board (or upgrading it to then unheard-of speeds and prices)? Make the CPU run at a multiple of the bus speed. Thus the clock multiplier and the DX/2 (and later 4 and 5) was born. The CPU ran at twice the speed of the bus, so a 486DX2-50 had a 50MHz internal clock, but a 25MHz external (bus) clock, and a 486DX2-66 had a 66MHz internal clock and a 33MHz external bus clock. That's what that manual is referencing.

Of course, running the CPU at twice the bus speed meant that suddenly a lot of operations started to bottleneck on the now-slower bus clock, so a 486DX2-50 was always going to be slower than a 486DX-50, but in practice most tasks were so heavily dependent on raw CPU (ALU) performance that the hit from running the bus at half speed was only a few percent. A 486DX2-50 might have been slower than the 486DX-50 but the 486DX2-66 was faster in almost all applications. And both were far more stable than the 50MHz-bus monster. So the multiplier was a success and pretty much every CPU these days has a very significant multiplier. The performance costs have been mitigated primarily by fast caches at full CPU speed (L1 on the 486 and Pentium, later L2 and L3 as well) and later integrating the memory controller onto the CPU so RAM could be accessed faster than the system bus would have allowed.

So yes, your DX2-66 would be exactly the "66MHz (internal)" CPU the manual is referring to, and to run it at spec the motherboard needs to be set to 33MHz.
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Re: What is Internal vs Internal MHz

Postby gdjacobs » 2018-4-19 @ 15:10

dionb wrote:This is about clock doubling (DX2 / 2x multiplier).


And clock tripling, although that's not mentioned by his manual.
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Re: What is Internal vs Internal MHz

Postby lazibayer » 2018-4-19 @ 15:35

GabrielKnight123 wrote:Im looking to getting the fastest CPU for my 486 motherboard and when I found a manual online from here:

http://www.arvutimuuseum.ee/th99/m/E-H/33912.htm

its says "25/33/40/50(internal)/50/66(internal)MHz" for processor speed but on the motherboard it only says "25MHz, 33MHz, 40MHz and 50MHz"

The CPU types the Mobo accepts are:
CX486S(WB)/CX486S(WT)/CX486S2(WB)/CX486S2(WT)/80486SX/ 80487SX/CX486DX(WB)/CX486DX(WT)/CX486DX2(WB)/CX486DX2(WT)/80486DX/80486DX2


50: board works at 50MHz, CPU works at 50MHz

50 internal: board works at 25MHz, CPU works at 50MHz


GabrielKnight123 wrote:I found an i486 DX2 66MHz would this be ok to use?


Yes
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Re: What is Internal vs Internal MHz

Postby GabrielKnight123 » 2018-4-20 @ 20:05

At the moment I have the jumpers set to use a 486SX 33MHz CPU do I leave the settings alone for the 486DX2 66MHz CPU to work?
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Re: What is Internal vs Internal MHz

Postby lazibayer » 2018-4-21 @ 05:02

You set the jumpers according to the manual's 80486DX2 settings.
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Re: What is Internal vs Internal MHz

Postby GabrielKnight123 » 2018-4-21 @ 08:49

Thanks Lazibayer I forgot about those ones but since the motherboard does not have a jumper setting for 66MHz do I keep the 33MHz jumpers as they are to get the 66MHz like what you said:

50: board works at 50MHz, CPU works at 50MHz
50 internal: board works at 25MHz, CPU works at 50MHz
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Re: What is Internal vs Internal MHz

Postby GabrielKnight123 » 2018-4-21 @ 11:32

I just realised something, because of the internal settings could I pop in a 100MHz CPU in and use the 50MHz motherboard settings to get the 100MHz if its a DX2? I already bought the 66MHz CPU but if the above is true I will get the 100MHz asap to get the fastest CPU this motherboard can use.
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Re: What is Internal vs Internal MHz

Postby jesolo » 2018-4-21 @ 11:39

Your motherboard appears to only support 5V CPU's - unless you can obtain a 486DX4 Overdrive CPU or a Pentium Overdrive (P24T), I would say that the DX2-66 is the fastest you can go with this motherboard.
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Re: What is Internal vs Internal MHz

Postby jesolo » 2018-4-21 @ 11:43

GabrielKnight123 wrote:but since the motherboard does not have a jumper setting for 66MHz

It does - it is the 66i MHz CPU speed setting (which is the same as the 33 MHz CPU speed setting).
For the other settings, make sure it is jumpered for the (Intel) 486DX2 settings.
Incidentally, the CPU type jumper settings for the 486DX & 486DX2 is the same.
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Re: What is Internal vs Internal MHz

Postby GabrielKnight123 » 2018-4-22 @ 12:03

Thanks Jesolo I have a 66MHz CPU coming to me soon I will use that one I also have another 486 CPU I will one day hunt a motherboard for it but the 66MHz will be good enough till then.
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Re: What is Internal vs Internal MHz

Postby Disruptor » 2018-4-23 @ 01:28

I guess the fastest CPU for your board is a Cyrix or ST DX2 80 MHz with 5 Volt and without Standard Pinout.
The reason is that this is the only CPU for your motherboard that can run internal cache in Write Back mode.
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