Effect of Turbo button on 386 and 486 CPUs

[dvanaria]

I've been reading that the Turbo button can slow down the system in several ways:

1. Slowing the CPU clock speed directly
2. Shutting off the CPU's internal cache
3. Introducing memory wait states.

My question is, is this all a function of the CPU itself? Or is it a function of the motherboard's chipset, BIOS, etc?

I'm thinking it's the BIOS, because I've heard of some systems being able to slow down when issued a keyboard command like CTRL + ALT + '+'

 

[badmofo]

Yes it's the motherboard doing that, and not all methods are equal from my experience. The best is actually underclocking the CPU, the rest can result in 'jerky' behavior in games. I have an OPTi board which underclocks the CPU, but it can only do that before EMM386 is loaded!

 

[orion24]

There were too many motherboards out there at the time and probably not a standard way of what the "turbo" is. So depending on the board you might get it in a different way. My 386 eg had the turbo enabled/disabled with a button on the case. While "opened" it was using the CPU clock, while "shorted" it was switching to the ISA clock for the CPU. Pure hardware way, nothing to do with its BIOS and no way to control it by software.

 

[smeezekitty]

Ony both the 486 I have had recently, they used a CTL ALT +/- to switch turbo on or off. No case button.
Since the results are all over the place in benchmarks I suspect that it cuts the cache

 

[Chuck(G)]

I've never found turbo/non-turbo mode switching more than just a gimmick in anything faster than a 286. The big reason is that later CPUs have more sophisticated instruction execution, so that instruction times don't line up even if you could slow a 486 to a speed comparable to a 4.77MHz 8088. Turbo mode seems to be popularized by Taiwanese manufacturers without offering a clear explanation of what it's good for. Pretty LEDs and 7-segment displays look cool, I guess. :roll:

 

[orion24]

Ony both the 486 I have had recently, they used a CTL ALT +/- to switch turbo on or off. No case button.
Since the results are all over the place in benchmarks I suspect that it cuts the cache

In 486 is is possible that you can enable/disable the cache with software and from the keyboard and call this "turbo". But this might be irrelevant to the "turbo" button some cases have. Even if your case has no such button, the switch might exist on the mainboard. That one probably switches from PCI clock to ISA clock for the CPU, making it operate at ~8MHz x CPU multiplier in non-turbo mode.

 

[High_Treason]

That JK-042A I have (486) can actually run slower than an IBM 5150 in theory but it does have an abnormal amount of options for the deturbo which is why this is possible. That one works from both button and keyboard. With my 40MHz setup it changes to an effective clock of around 2.5MHz and I have it set to disable cache (internal and external) among other things (can alter clocks / wait-states for other things) - It runs about 1/4 the speed of my 6MHz 286 according to all benchmarks and games that I tested under the current configuration.

As far as I know, switching is controlled somewhere with the Keyboard Controller, BIOS and Chipset though beyond that I have no idea.

A little OT but I discovered whilst fiddling with the BIOS on a VIA MVP3 Super 7 board I was using that said board has a sort-of-working Deturbo function that was not connected to a pin on some chip (I forget which, BIOS I think) like they appear to have intended, though the front panel header included connections for a switch and LED that did nothing as did the keyboard shortcut, as far as I could tell that also turned off cache and switched the RAM to a different clock (PCI or AGP, unsure) but the CPU clock begins fluctuating and crashes... Cheap no-name board, no idea who made it, even the manual doesn't say so you know they were proud of their work.

To my knowledge, common keyboard shortcuts are
Fn + F11/Fn + F12, CTRL + ALT + - / CTRL + ALT + + will enable / disable Turbo, wheras CTRL + SHIFT + ALT + - and CTRL + SHIFT + ALT + + will turn cache on or off, though I have no boards with the cache shortcut working anymore, guessing it was rarely implemented.

 

[vwestlife]

My CompuAdd 810 has a three-step speed setting, controllable via the keyboard:

Ctrl+Alt+[*] = 4.77 MHz
Ctrl+Alt+[-] = 7.16 MHz
Ctrl+Alt+[+] = 9.54 MHz

 

[Chuck(G)]

I believe that the original reason for the turbo button arose from a lot of 8088 code using CPU loops to provide delays. An example of this can be found in some BIOSes that used a programmed loop to delay accesses to the FDC (the datasheet for the 765 specifies how long a delay (50 usec, IIRC) between starting a command and reading status, for example. So running a program that performed delays like that on a fast system could cause the software to fail.

An example is the original version of Fastback.

 

[barythrin]

I had at least one 386 laptop that used ctrl+alt+h or ctrl+alt+l (high or low performance).

 

[vwestlife]

I had at least one 386 laptop that used ctrl+alt+h or ctrl+alt+l (high or low performance).

And Tandy computers used MODE FAST or MODE SLOW at the DOS prompt (using the original Tandy version of MS-DOS), or you could hold down a function key during POST (varied depending on model) to start up in slow speed.