Re: Accelerator / VGA Graphics card

[mjmahon@aol.com (Michael J. Mahon)]

I earlier wrote:

>Sheldon Simms replied:
>
>>On Fri, 24 Oct 2003 21:56:34 +0000, Michael J. Mahon wrote:
>>
>>> Sheldon Simms wrote:
>>> 
>>>>On Fri, 24 Oct 2003 14:59:39 -0400, Charlie wrote:
>>> 
>>> <snip>
>>> 
>>>>> That makes sense and I like the idea of the 1MHz clock generating an
>>>>> interrupt on the card.  Wouldn't this free up the emulator to do other
>>>>> things (graphics) in between interrupts?
>>>>
>>>>Well yes, but it would probably do everything it needs to do and then
>>>>still be waiting around.
>>> 
>>> Have you checked out how long the Pentium M requires to take an
>>> interrupt and restore state and return?  I'll bet that caches are much
>>> less helpful for this than you imagine, and _lots_ of cycles are needed.
>>
>>After reading this I poked around a bit in Intel documentation but
>>they are not very forthcoming. However, considering the front side
>>bus frequency of 400Mhz, and the fact that all code on the Pentium M
>>would be running in supervisor mode (i.e. - no need to change protection
>>level), I have a hard time believing that it couldn't handle a 1Mhz
>>interrupt source. But perhaps I'm wrong. In that case other measures
>>would be required.
>
>Note that FSB speed only applies to burst transfers from RAM to and
>from cache, and does not include the startup latency, which is probably
>50-60ns.
>
>I'm not saying that the processor can't handle interrupts at a 1MHz
>rate, but I am saying that it may use a lot bigger fraction of the
>processor's time than a first reading of the specs might suggest.
>
>Today's high speed processors do their best work on the straightaway
>and slow down mightily in the "curves".

<snip>

I thought I should add that I don't think this need be a problem in any
event, since you will only need a 1MHz (Phase 0) interrupt while you
are running in "slowed down" mode interfacing with the Apple II bus.

It should be unnecessary to take 1MHz interrupts while running at
high speed (though some video generation interrupts may be useful).

-michael

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