Authour: Alex 'AlexV' Voicu
Editor: Charles 'Lupine' Oliver
Date: November 13th, 2008
In order to clear potential misconceptions, we'll introduce the HD3300 by stating that the it is simply an overclocked HD3200 (the IGP in the 780G). The overclock, however, is a rather hefty 200MHz, so the bonus in performance should be consistent. In case you were wondering what's under the hood, take a peek:
The HD3300 is functionally a carbon copy of the RV610 (HD2400), albeit shrunk to 55nm, so you'll get all of the above goodies packed into your northbridge should you opt for a 790GX based motherboard. Differences appear once we start looking at how memory is dealt with, since an IGP has to rely primarily on system RAM:
To access RAM, the IGP has to go through the Integrated Memory Controller (IMC) on the CPU, with which it communicates via the HT-link between the northbridge and the CPU - there's a latency cost associated with doing this.
If Sideport memory is present, and this isn't mandatory and is up to the motherboard maker to include it or not, it is accessed via a 32-bit wide physical connection - this means lower latency, as well as avoidance of possible congestion of the CPU-NB link or of the IMC; this is pretty much identical to how discrete graphics cards behave.
When using both main RAM, Unified Memory Architecture (UMA), and the Sideport, there are 2 possible operating modes:
Interleaving enabled: this allows the IGP to request data from the UMA as well as the Sideport in parallel, equating with the possibility of 2 simultaneous reads/writes, as opposed to normal operation when only one 32 bit read or write can occur; for initial Sideport implementations, UMA size equaled Sideport size, but more fine-grained control is now possible, and different Sideport:UMA ratios are allowed.
Interleaving disabled: total memory for the IGP is equal to UMA size + Sideport size, with Sideport memory being used preferentially until filled, after which system RAM is utilized.
All the 790GX boards we've seen allow control over interleaving.
As you'll soon see though, the HD3300 doesn't pack enough processing power to be really sensitive to memory access latencies/bandwidth outside of scenarios where low resolutions/quality settings are employed. But before that, let's dissect the second novelty.
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