VOGONS


First post, by megatron-uk

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Is there a standard rule-of-thumb for a 386 (SX or DX) when turbo is not activated? Is it always at the 14.318MHz clock, half the usual processor clock, or was it just completely dependent on the motherboard implementation?

My experience with 286 boards is that they are usually 1/2 the nominal processor clock.... but then by the time the 486 came along it seemed that it was all implementation-specific. I had a 386 (SX) for a brief period of time back in the early 90's, but until recently I didn't really spend much time with them.

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Reply 1 of 5, by Doornkaat

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I'm not sure I understand the question.
386 CPUs come in a variety of speeds. The external (fsb) clock is the same as the internal clock. If the motherboard uses an oscillator to generate the CPU clock it's usually twice the CPU clock.

Reply 2 of 5, by megatron-uk

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Yes, I understand. However, what I'm wondering is what the usual clock-speed behaviour is when turbo is not activated and the processor is not running at the normal on board oscillator speed (usually either 1:1 or 1/2 frequency of the can)?

On most (not all) 286 designs it drops to 1/2 of the incoming processor clock, on a 486 it's very much motherboard implementation dependent.

I was wondering if it was more common to be the former, or the latter.

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Reply 3 of 5, by Doornkaat

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Ok, now I understand!😄
From my experience the slowdown method and speed of 386 system's (de)turbo implementation seem to vary from board to board.
I noticed that a common implementation seems to be using two oscillators, one at double the nominal CPU frequency and one at double the slowed down CPU frequency. The turbo button seems to switch between the oscillators.

Reply 4 of 5, by megatron-uk

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Ah - so more like the behaviour I observe on the 486 than the 286.

Good to know.

The underlying reason is to try and figure out whether I should configure the 'speed' display on my recently acquired case for my 386 project(s) to a static value (e.g. 40 high / 40 low) or to set it to a pair of values representing the actual non-turbo speed. Sounds like the latter isn't practical (and setting the correct jumpers on these things is frustrating enough once!).

My collection database and technical wiki:
https://www.target-earth.net

Reply 5 of 5, by Deksor

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Many 386 SX boards I have even go as far as offering multiple dividers parametrable within the bios (1/1, 1/2, 1/3 and 1/4)

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