Turbo Button & 486 Slowdown

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Turbo Button & 486 Slowdown

Postby Great Hierophant » 2011-6-01 @ 02:50

How slow can you make a 486 go?

Most 486 motherboards, at least the OEM/sysem builder boards, support a turbo button. As I have read, the turbo can do one of three things to slow down the machine:

1. Cut the processor speed to 8Mhz

2. Insert wait states to simulate 8MHz speed

3. Turn off the cache

#3 seems to be the least useful in the face of programs that can do the same thing. However, you may not be able to activate and deactivate those programs in the midst of a DOS game.

#2 is more interesting, as a large portion of the processor's time deals with reads and writes to memory. But this only slows down the memory accesses. Operations inside the processor still should occur at the faster speed.

#3 seems the most effective of the methods and perhaps the rarest. It would give a proportional slowdown across the board. These slowdowns would typically go down to 8Mhz so the ISA bus would not be affected. I do not know if 486s generally would run at 8Mhz.

Additionally, as boards allowed the user to set the speed, you could also run the processor at a lower speed. The slowest 486SXs ran at 16MHz, but many boards only go down to 20MHz or 25Mhz. I believe the DX2 and above multiply the clock inside the processor, so it should always run double the motherboard's set speed.

Of course, you can may be able to combine the lower speeds with the turbo, depending on what your motherboard and processor will support.

So, there are many 486 chipsets out there, SiS, VIA, Aladdin, OPi etc. Does anyone know which turbo method is used on which chipsets?

Can anyone provide comparisons or benchmarks of underlocked or slowed down processors.
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Re: Turbo Button & 486 Slowdown

Postby iulianv » 2011-6-01 @ 05:02

Is the method chipset-related or manufacturer/mainboard-related? I have an OPTi-based VLB board that seems to "de-turbo" by turning off cache...
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Re: Turbo Button & 486 Slowdown

Postby 5u3 » 2011-6-01 @ 20:25

Image

This is my 486 at 8 MHz. The board is a PVI-486SP3 (SIS 496/497) with the turbo still on.
Turning the turbo off is not as effective as either disabling the cache or running the system at 8 MHz, so it has to be the waitstates method.
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Re: Turbo Button & 486 Slowdown

Postby retro games 100 » 2011-6-01 @ 21:33

Great Hierophant wrote:Can anyone provide comparisons or benchmarks of underlocked or slowed down processors.

I ran some tests using the same mobo that Vogons user 5u3 has. I altered its bus speed, cache (on/off), turbo (on/off), bios settings (fast/slow). I ran 3DBench for each setting, and reported the results in a big table here. (about half way down the page).

Using undocumented bus speed jumper settings, as 5u3 says "...running the system at 8 MHz...", you can get this mobo to run that slowly. Also, Vogons user BastlerMike mentioned in this thread, that FIC mainboards also have the ability to run their CPUs at very slow speeds using various bus speed jumper settings.

@GH, why are you interested in slowed down 486s? Do you have speed sensitive games that don't run perfectly on your current systems?
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Re: Turbo Button & 486 Slowdown

Postby Great Hierophant » 2011-6-03 @ 01:04

5u3 wrote:Image

This is my 486 at 8 MHz. The board is a PVI-486SP3 (SIS 496/497) with the turbo still on.
Turning the turbo off is not as effective as either disabling the cache or running the system at 8 MHz, so it has to be the waitstates method.


It looks like the program seems to equate the 486@8Mhz to be slightly slower than the 386DX40. But what would happen if cache was turned off and/or the turbo was off at that speed? Is it possible to slow the machine down to AT (286/8) speeds in hardware?
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Re: Turbo Button & 486 Slowdown

Postby DonutKing » 2011-6-03 @ 02:50

It looks like the program seems to equate the 486@8Mhz to be slightly slower than the 386DX40.


Just FYI my 386DX-40 scores about 9.2 in SPEEDSYS. With the turbo button disabled it just seems to introduce wait states.

I do have a 12MHz 286 board here, and when the turbo button is disabled Norton Sysinfo reports that its clockspeed changes from 12MHz to 8MHz and back again when the turbo is enabled. None of my other 386/486boards do this; the CPU score changes but the clockspeed stays constant on screen.
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Re: Turbo Button & 486 Slowdown

Postby Great Hierophant » 2011-6-03 @ 04:05

DonutKing wrote:
It looks like the program seems to equate the 486@8Mhz to be slightly slower than the 386DX40.


Just FYI my 386DX-40 scores about 9.2 in SPEEDSYS. With the turbo button disabled it just seems to introduce wait states.

I do have a 12MHz 286 board here, and when the turbo button is disabled Norton Sysinfo reports that its clockspeed changes from 12MHz to 8MHz and back again when the turbo is enabled. None of my other 386/486boards do this; the CPU score changes but the clockspeed stays constant on screen.


What does 1.0 in Speedsys represent? An 8088@4.77Mhz? A 286-8?, a 386SX16?
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Re: Turbo Button & 486 Slowdown

Postby Great Hierophant » 2011-6-03 @ 04:06

Great Hierophant wrote:
DonutKing wrote:
It looks like the program seems to equate the 486@8Mhz to be slightly slower than the 386DX40.


Just FYI my 386DX-40 scores about 9.2 in SPEEDSYS. With the turbo button disabled it just seems to introduce wait states.

I do have a 12MHz 286 board here, and when the turbo button is disabled Norton Sysinfo reports that its clockspeed changes from 12MHz to 8MHz and back again when the turbo is enabled. None of my other 386/486boards do this; the CPU score changes but the clockspeed stays constant on screen.


What does 1.0 in Speedsys represent? An 8088-5? A 286-8?, a 386SX-16?
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Re: Turbo Button & 486 Slowdown

Postby 5u3 » 2011-6-03 @ 09:23

Great Hierophant wrote:It looks like the program seems to equate the 486@8Mhz to be slightly slower than the 386DX40. But what would happen if cache was turned off and/or the turbo was off at that speed? Is it possible to slow the machine down to AT (286/8) speeds in hardware?


I don't know how the Speedsys processor benchmark is calibrated, but it is probably not very useful for comparing CPUs lower than 486.

A few things about the "8 MHz" screenshot: Both caches were turned off for this, because the CPU wouldn't post at 8 MHz with the caches activated. Also, I had to set the ISA clock to the asynchronous setting (7.something MHz), and I couldn't use any PCI cards.

Speedsys took 1.5 hours to draw the screenshot (maybe I should have removed most of the memory first :wink:).

Extreme slowdowns like this just don't seem very practical. I haven't tested many very old games yet, but usually a 486/25 without caches and an ISA VGA is slow enough to get most things going.

The turbo button sometimes is useful to get past speed-related quirks like some copy protection or shoddy hardware detection, but it's not very good for playing games since it also causes laggy screen updates, at least in the way it's implemented on my board.
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Re: Turbo Button & 486 Slowdown

Postby DonutKing » 2011-6-03 @ 09:24

I have no idea what the Speedsys score is meant to represent but Norton Sysinfo's CPU score is normalized to a 4.77MHz 8088.
My 386DX-40 scores about 40 in that.
If I hit tthe turbo button that goes down to 8.
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