From: Paul Jakma (Paul.Jakma at domain Digital.com)
Date: Thu 20 May 1999 - 15:04:08 IST
> From: Colm Buckley[SMTP:colm at domain tuatha.org]
> Sent: 20 May 1999 13:49
> To: ilug at domain linux.ie
> Subject: Re: [ILUG] Re: Fonts...
> Paul Kelly writes:
> > Refresh rate is not related to the size of RAM - it's a function of
> > the RAMDAC.
> Yes and no. It's true that refresh rate doesn't depend on the size of
> the RAM per se, but there is a dependency. Most RAM chips have a
> maximum speed - from serving out the contents of location X, they need a
> short time to recover before they can serve out the contents of location
> Y. Fast memory subsystems get around this by using multiple banks of
> RAM, so that successive locations are read from different chips - eg :
> location 0 might be from bank 0, location 1 from bank 1, location 2 from
> bank 2, location 3 from bank 3, location 4 from bank 0, location 5 from
> bank 1, and so on.
> Although it's not a hard-and-fast rule, 8Mb cards typically only have
> one or two banks of RAM, whereas 16Mb cards have two or four - hence the
> maximum dot-clock is related tangentially to the size of your RAM -
> it's really dependent on the overall speed of your graphics memory,
> which in turn depends on the number of RAM banks you have.
I think the latency of RAM access is more of an issue to keeping the
graphics engine fed than anything else. Which is why modern cards are so
heavily pipelined, eg the G200. Then the graphics engine outputs to the DAC,
so wouldn't it be something like this:
(hope this comes out right)
| bus to
The graphics engine is such a huge buffer that i guess it would effectively
isolate RAMDAC from framebuffer performance. And the graphics engine i
guess has dedicated circuitry for feeding the DAC - at least i've never
heard of graphics cards 'dropping frames' - even if the FPS in your
favourite rate 3D game goes down, the DAC is still consistently pumping out
a signal to the monitor at the configured refresh rate. (if it didn't it'd
probably fry a lot of monitors).
Of course if you using a large resolution at a high colour depth you need a
higher MHz RAMDAC to cope with the bandwidth, but that's related to the
amount of RAM, not the speed..
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