VOGONS


First post, by Robhalfordfan

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hello all

i am currently working on my 486 build

the cpu i am using is 486 dx2 - 66

when i push the turbo button - it goes from 66 to 08 - that doesn't seem right to me

is there a dos program that can actually tell me what speed the cpu is when the turbo button is pushed

Reply 1 of 15, by PARKE

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Speedsys shows a relative speed, in this example 110Mhz versus 133Mhz.

NOTURBO.jpg
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Turbo does in general not affect the cpu speed/frequency but fiddles with cache etc.

You can dwnload the program here
https://drive.google.com/file/d/0B2BOLB8yC3mn … GdGYmNjSzg/view

Reply 2 of 15, by Zup

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Robhalfordfan wrote on 2020-09-28, 14:15:

when i push the turbo button - it goes from 66 to 08 - that doesn't seem right to me

Where did you saw that? Just remember that (most) speed displays on cases can display whatever you want as long as you can put on 7 segment displays (i.e.: 66/33, HI/LO, those things) and does NOT show anything related to actual speed.

PARKE wrote on 2020-09-28, 14:54:

Turbo does in general not affect the cpu speed/frequency but fiddles with cache etc.

I always believed that turbo buttons told the mainboard to switch between two FSB settings...

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Sometimes going all the way is just a start...

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Reply 3 of 15, by chinny22

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Zup wrote on 2020-09-28, 15:05:
PARKE wrote on 2020-09-28, 14:54:

Turbo does in general not affect the cpu speed/frequency but fiddles with cache etc.

I always believed that turbo buttons told the mainboard to switch between two FSB settings...

Everyone is correct
Was never a "Turbo standard" was up to the manufacture how they wanted to implement it (if at all)

Reply 4 of 15, by PARKE

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Zup wrote on 2020-09-28, 15:05:
PARKE wrote on 2020-09-28, 14:54:

Turbo does in general not affect the cpu speed/frequency but fiddles with cache etc.

I always believed that turbo buttons told the mainboard to switch between two FSB settings...

See this thread:
How does the Turbo Button work?

Reply 6 of 15, by digistorm

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Interesting… on my 486 DX2/66 Speedsys always reports 22 MHz, with or without turbo 🤣 (22 ís in line with the manual, that says that the turbo button reduces the clock with ⅓, whatever that means)

Reply 7 of 15, by Robhalfordfan

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here is screenshots from speedsys after wiping the hdd and reinstalling dos 6.22 and from a clean boot

the ram timing/ wait states etc are all at default settings in the bios

top one is when the turbo button not pushed
middle is when the turbo button is pushed
bottom is bios defaults settings

if someone can explain this to me and what it all means as i am noob with this era of pc's

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  • 486-66.jpg
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    486-66.jpg
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    full 66mhz - not pushed
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  • 486-SLOW.jpg
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    turbo button pushed
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  • DSC_1776.JPG
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    bios settings
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Reply 8 of 15, by Baoran

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Speed when turbo is disabled depends on the motherboard. When it comes to my 486 motherboards I have noticed that older the motherboard the more it slows down compared to newer motherboards. The turbo display does not matter at all because motherboard does not tell the display what to show. You can generally set it yourself what the mhz display shows when turbo is on or off. You could make it show 99Mhz when turbo and 1Mhz when turbo off no matter what cpu you have if you wanted. If you want the display show correctly you pretty much have to estimate yourself how much slower the pc is. In your case based on those speedstep results you could set the display to show 66Mhz when turbo is on and 30Mhz when turbo is off. The clockspeed does not generally change in reality, but you can set the display based on slowdown percentage that you are getting from it.

Reply 9 of 15, by debs3759

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The Turbo button was originally added to reduce the CPU speed for faster 808x chips that some software couldn't handle. There was never s standard for how it would act on faster processors, it was only ever documented (rather counter intuitively) as slowing the CPU.

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Constantly being worked on. Feel free to message me with any corrections or details of cards you would like me to research and add.

Reply 11 of 15, by pan069

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Robhalfordfan wrote on 2020-09-28, 21:16:

how can i tell what speed is, so i can get the turbo display on case to shows the correct speed by trial and error with jumpers

That is correct. You could e.g. configure the display to show HI or LO. This is what I do on my 100Mhz 486 where the turbo display only has 2 segments so I can't fit 100 on it, i.e. I opted for HI (turbo on) and LO (turbo off).

Reply 12 of 15, by Baoran

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Robhalfordfan wrote on 2020-09-28, 21:16:

how can i tell what speed is, so i can get the turbo display on case to shows the correct speed by trial and error with jumpers

At full speed your speedsys result was over 25 and when turbo is disabled it was 12 which is bit less than half of full speed of 66Mhz. That is why I suggested that you could set the display to show something like 30Mhz when turbo is disabled.

Reply 13 of 15, by kdr

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As everyone has already mentioned, there's no "standard" way to implement the turbo/non-turbo mode functionality.

For example, the VLSI VT82C486 chipset (for 486DX and 486DX2 class CPUs) has this to say about its implementation:

TURBO/NON-TURBO MODE CONTROL

It has become standard for fast PC/AT-compatibles to provide means to slow operation for older speed sensitive software. This is especially true for graphics intensive entertainment software which may otherwise operate much too fast on a high speed machine. One way this mode may be toggled on and off is by external control of the TURBO input pin. The Slow Mode is activated and the VL82C486 generates continuous invalidates (via -EADS) to the CPU when TURBO is low. When TURBO is high, the -EADS signal is not modulated. This feature provides the capability of emulating the effect of running at slower CPU clock frequencies even though the clock frequency is not changed. It is provided in addition to a method of actually changing the CPU clock frequency.

The TURBO pin is normally connected to the keyboard controller and triggered by the BIOS via detection of a key combination such as Ctrl Alt + / Ctrl Alt -. This input is often externally ANDed with a mechanical Turbo switch on the front panel.

So the VLSI implementation activates the -EADS signal of the 486 CPU, which causes it to execute an "invalidate internal cache" cycle instead of whatever normal cycle it would otherwise execute. Basically stealing clock cycles from the 486 by keeping the external bus occupied with bogus invalidate cycles. It offers a choice of how much to slow down the CPU, too: full speed (no holds), 50% (a hold every other clock cycle), 66.6%, ... all the way up to 87.5% (seven holds for every eight clock cycles).

Reply 15 of 15, by Robhalfordfan

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to give an update with speedsys

everything is set the same - The only different is that i have set the wait states in bios to 0 instead of the default 1

compare to the previous results from earlier post

deffo a different at "full" speed with cache and memory throughput and bandwidth
with "slow", there is a different, maybe not a great deal but speedsys reports no l2 cache unlike with 1 wait states in "slow" mode

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  • NOWAIT-F.jpg
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    NOWAIT-F.jpg
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    "Full" mode- No Wait States
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  • NOWAIT-S.jpg
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    NOWAIT-S.jpg
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    "Slow" mode - No Wait States
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