Re: Accelerator / VGA Graphics card

[Sheldon Simms <sheldonsimms@yahoo.com>]

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.

>>> And even if there was nothing to do, from the little I've read about
>>> the Efficeon chip it drastically reduces its power consumption when
>>> it idles.
>>
>>So does the Pentium M. This sort of idling won't save too much power
>>though, because it wouldn't happen except when the card needed to
>>access the bus -- disk, DOC, flipping soft switches, etc. And
>>most programs spend only a small amount of time doing that stuff.
> 
> And if you speed the processor execution up by a factor of, say,
> 25x, then the time spent doing these low-speed things does not
> accelerate, and the fraction of time doing low-speed things rises
> proportionately.
> 
> For example, if a real Apple II running program X takes 10% of its
> time doing things like disk I/O, then even if the processor is
> accelerated infinitely, the program runs only 10x faster, since
> all of its time is now spent on disk I/O.

Of course, but few real Apple II running programs spend 10% of their
time doing disk I/O. In fact, most Apple II programs amenable to
acceleration don't do disk I/O at all except at occasional well-
defined times (such as when the user saves a document).

I don't deny that "slowing down" to do I/O puts a brake on the
accelerator, but it does that for acclerators like the Zip GS or
Transwarp GS as well. I expect the performance of this proposed card
would greatly exceed the performance of previous accelerators
regardless.

-Sheldon