DDR memory's affect on AGP



Will the increased bandwidth from ddr system memory make the ability of agp video cards to store they're textures in system memory better?
because the system memory will be running at almost the same speed as the video card memory (assuming you have 266 MHz ddr memory and a geforce2 with 333 mhz memory...
will that make it almost like having an extra 64/128Mb ram or so on the video card?


I wouldn't say it would be like having extra memory on the video card. Video card memory is *MUCH* higher bandwidth than system memory anyway, so it's sort of the video subsystem equivalent of processor cache (I know that's a bit of a kludged explanation, but you get the idea).


yeah you are right there about bandwidth. Most video products that use DDR ram is at 64bit wide data path, some even 128bit (nvidia gf ddr ?). Whereas i'm pretty sure that system memory data width is only(?!) 32bit.

Ian McGinley


Effect on AGP is not in anyway affected by what
the videomemory is. AGP bus is way out of the leaque here.
How will the DDR system affect on AGP performance is that
on 4x mode the system-memory will finaly be fast enough to
fill the AGP bandwidth in reallife situations. So, it will
help some. But not to increase the amount of USABLE amount
of textures and so on.
That is because if you have more textures than the
video-memory can hold, you would have to update those for
EVERY frame and you can't do much of that kind of swapping
with AGP. That is, one COULD do it, but it would eat all
the bandwidth from cpu and rest of the computer, probably
causing a big drop in FPS. Which is clear nobody wants.

What one can do with the bandwidth, is replacing the
texture allready in video-memory, to do like animations,
which can run on independant speed compared to FPS on
screen. Or one can use more complex object or animate
objects. But as the T&L (i hate that term, it's GPU)
is allready being increasingly used, the objects will
allso be kept mostly in video-memory.

Oh and for the record, one can write to video memory fast,
but reading is still slow.


Until AGP 8x speed comes out, the additional bandwidth offered by DDR SDRAM will still be underused. Video RAM is still far more preferable than system RAM for storing texture maps - comparing extremes, the bandwidth offered by a standard GTS Ultra is 6400Mb/sec whereas the AGP bus at 4x is only 1064Mb/sec. The bus is still too much of a bottleneck, even with 8x speed (2133Mb/sec).

Bus widths go as follows:

All GeForce cards (with the exceptions of the MX series) = 128 bits = 16 bytes
FSB for Pentium 3 and Athlon chips = 64 bits = 8 bytes
Memory bus for SDRAM and DDR SDRAM = 64 bits = 8 bytes
Memory bus for RDRAM = 16 bits = 2 bytes
AGP bus = 32 bits = 4 bytes

Maximum theoretical bandwidths are calculated by multiplying the clock speed by the bus width.

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