Re: Accelerator / VGA Graphics card

[Sheldon Simms <sheldonsimms@yahoo.com>]

On Sat, 25 Oct 2003 06:51:35 +0000, Michael J. Mahon 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.

Well another option would be to have a bus-width bank of shift
registers (a shift-memory, if you will). The emulation code writes
the entire bus transaction out to this memory which is then clocked
out onto the bus by the 1Mhz clock.

> If you're into computing for speed, Apple II's are a terribly
> inefficient way to get there.  ;-)
> 
> And if you're into computing for fun, Apple II's will get you
> there at a very enjoyable speed.

Well fun is the point of this card too. I'm not planning on replacing
my PC with an 80 Mhz IIgs...

-Sheldon