SidePort: On-Board GPU Memory

Just before the launch of the Athlon 64, we found that many chipset manufacturers were a bit worried about the performance of integrated graphics solutions and AMD's new CPU. The worries stemmed from the fact that in previous CPU/chipset architectures, the integrated graphics cores resided on the North Bridge and shared access to the system memory controller - also located on the North Bridge. With the Athlon 64, however, the memory controller resides in the CPU, increasing memory access latencies from the perspective of the integrated graphics core.

The Radeon Xpress 200 supports a local frame buffer attached to what ATI refers to as their "SidePort". The SidePort is a 32-bit DDR memory interface that the integrated graphics can use either instead of or alongside the Athlon 64's memory controller.

While we assumed that SidePort was included to hide some of the latencies of using the Athlon 64's memory controller, that ended up not being true as performance in UMA mode (using the Athlon 64's memory controller) was quite respectable. It turns out that most games don't benefit too much from lower latency memory accesses (through SidePort). So, why would ATI include support for a local frame buffer with the Radeon Xpress 200? Although performance is improved with SidePort enabled, the biggest reason for supporting the feature is to reduce power consumption in mobile environments. Without SidePort enabled, the CPU needs to be awake to fetch data for refreshing the display, but with SidePort enabled, all memory accesses can occur via the Radeon Xpress 200 and the CPU can remain asleep in power saving modes.

Because of the added cost of supporting SidePort, it isn't a requirement - the Radeon Xpress 200 has four memory operating modes:
  1. SidePort only - In this mode, the integrated graphics core treats the SidePort memory as its local memory. If more memory is needed, it is allocated dynamically through system memory by the driver, which is significantly higher latency than the local SidePort memory.
  2. UMA only - In UMA mode, the only memory to which the integrated graphics has access is a dynamically allocated partition of system memory. The size of the parition is selectable from within the BIOS (ATI's reference board allows for 16 - 128MB sizes). If more memory is needed, it is allocated dynamically through system memory by the driver.
  3. UMA + SidePort (Interleaving Disabled) - In this mode, the total amount of "local" graphics memory is the size of the UMA partition and the amount of memory connected to the Radeon Xpress' SidePort. The integrated graphics core will first use SidePort memory until it runs out, then using system memory. If more memory is needed, it is allocated dynamically through system memory by the driver.
  4. UMA + SidePort (Interleaving Enabled) - By enabling Interleaving and setting the UMA frame buffer size to the same size as the memory connected to the Radeon Xpress' SidePort, a special Interleaving mode is enabled. In this mode, the integrated graphics cores will request data from both the UMA space and SidePort memory. The benefit of Interleaving is that now two reads or writes can occur at the same time, whereas with just SidePort only a single 32-bit read/write can happen at any given time. Despite the fact that UMA accesses will be higher latency, the dual ported nature of this setup improves overall performance. There are situations when a SidePort only configuration will offer greater performance if the application depends on lower latency memory accesses. If more memory is needed it is dynamically allocated through system memory by the driver.
ATI recommends that if SidePort memory is present on the motherboard that the fourth configuration (UMA + SidePort with Interleaving Enabled) is used as it offers the highest overall graphics performance.

ATI's reference board features 16MB of DDR memory attached to the Radeon Xpress' SidePort. The memory can either run synchronously with the system memory clock (200MHz for DDR400) or asynchronously, where the speed is bound by the type of memory used. In our case, the 2.5ns Samsung DDR located on the board was capable of running at the maximum frequency the BIOS allowed - 350MHz.

As we mentioned before, the SidePort memory interface is a single 32-bit channel, which at 350MHz provides 1.4GB/s of bandwidth to the integrated graphics core. At 200MHz SidePort can only provide 800MB/s of bandwidth, so the additional latency incurred by running the SidePort asynchronously with main memory is well worth the additional bandwidth.

What's truly interesting is the pretty impressive performance of running in SidePort-only mode. Granted you are limited to low resolutions, but as you will soon see, the integrated graphics core isn't really designed to run at very high resolutions. In fact, running in SidePort-only mode is faster than running in UMA only mode with a single-channel Socket-754 Athlon 64.

The charts below do a good job of showing off the performance advantages to the various operating modes of the Radeon Xpress 200.

The first thing we see is that there's a huge performance advantage to the dual channel memory controller of the Socket-939 Athlon 64, - 33% in Doom 3 and 27% in UT2004. This is far from unexpected given that the more system memory bandwidth you have, the more graphics memory bandwidth you have.

The performance advantage to using the SidePort + UMA configuration isn't insignificant either - 8.5% in Doom 3 and 7.7% in UT2004, however with the added cost we would say that the SidePort isn't absolutely necessary for desktops (but we understand its usefulness in notebooks).

Radeon Xpress 200 Memory Configuration Comparison

Radeon Xpress 200 Memory Configuration Comparison

We compared the graphics performance of the Radeon Xpress 200 to ATI's lowest end discrete PCI Express graphics card: the Radeon X300 SE. The X300 SE is a four-pipe version of the Radeon Xpress 200 but with only a 64-bit DDR memory interface, so the Radeon Xpress 200 actually holds a memory bandwidth advantage over the X300 SE while it is at a fill rate deficit.

We also compared the Radeon Xpress 200 to Intel's Graphics Media Accelerator 900. While the GMA 900 is obviously only available on the Intel-only 915G and the Radeon Xpress 200 is an AMD-only solution, the two offerings are slow enough that most games end up being completely GPU limited and thus the CPU differences become negligible.

You'll notice that not all of the benchmarks have scores for Intel's GMA 900; those that don't have GMA 900 scores are ones where the GMA 900 was not able to either run the game or complete the benchmark without crashing.

Radeon Xpress' Integrated DirectX 9 GPU Integrated Graphics Performance Comparison
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  • Sahrin - Monday, November 8, 2004 - link

    I'm really excited to see another performance player in the AMD chipset market. Ironically, despite the fact that Intel is considered to have the best quality chipsets, the AMD segment has the most players and the most options. This chipset looks very good to me, especially as an overclocker, but I'm kind of left hanging in the feature set, which traditionally has been the determinant in the A64 market. Sure, 6 SATA ports is nice...etc. etc. but where's my dual integrated GigE LAN? I will take a long hard look at this chipset if SB450 comes out in time, but I think I will likely be going nForce one more generation. Reply
  • SLIM - Monday, November 8, 2004 - link

    #6, of course you use an FX with the best gfx card available, he's trying to highlight small differences between chipsets. If you want P4 vs A64 look at a recent cpu review.

    However one large set of differences were the specviewperf benches? Huge differences when using ati/ati (some good and some bad) but no comments as to wtf is going on. Are those differences related to DX vs opengl, other driver issues, anybody know??
    Reply
  • ipoh - Monday, November 8, 2004 - link

    Onboard graphics use to be not good but changed since ATi comes out with RS350...and with this RS480 DX9 VGA will be definitely good

    Currently using my RS350 playing Doom3 and still looks good :)

    I will spend my money for more HDD :)
    Reply
  • Ivo - Monday, November 8, 2004 - link

    With DX9 included, the integrated graphics (IG) of RS480 is good. First of all, with guaranteed future OS compatibility, it's very good for the OEM - for both business machines and home-office PCs. Secondly, as stated it the review, it is good enough for high-end 2D users because of the Surround View option. Third, it is a reasonable option for gamers too, as it could serve in emergency cases, when your high-end overclocked graphic card is tired ;-(

    The IG could be even more interesting for occasional gamers and even business users if, in a thinkable upcoming chipset, the IG is involved in a SLI scheme with one graphic card. In that case the IG will add it's modest 10% to 20% to the overall gaming performance (small, but from heart). This 10%-20% could be interesting for the real gamers too, if the IG is involved in a triple SLI scheme with two additional graphic cards.

    My questions to this great article are:
    1. What about the Cool 'N Quiet operation - does it work properly on the reference board with all (DIMM etc.) configurations used?
    2. What is ATI suggesting about the SidePort - why it is limited to 32 bit and 16MB only?
    Reply
  • byvis - Monday, November 8, 2004 - link

    It's very impressive. But I have one minor question about the benchmarking. Why didn't you test Nforce4 + X800XT in Winstone and other benchmarks? I see, that you DID test RX480 + GF6800U and RX480 + X800XT. Maybe the margins are very small, but I'd like to see them, I think other people would like that too. Reply
  • deathwalker - Monday, November 8, 2004 - link

    ATI might be right in the thick of it based on performance..however...from a marketing standpoint I think they will have a tough road to plow. Reply
  • bearxor - Monday, November 8, 2004 - link

    Sold Reply
  • Jalf - Monday, November 8, 2004 - link

    Onboard graphics makes perfect sense for non-gamers.
    If they can cram in something that works for normal desktop use, *and* can claim to support DirectX 9 as well, then it's a pretty good deal. It'll serve your needs under normal use, and it'll at least be able to run games, even if they might get an unplayable framerate.
    Reply
  • DrDisconnect - Monday, November 8, 2004 - link

    I'm surprised that any of you are wondering why they are producing an integrated graphics versio. Haven't you taken a walk through any of the computer superstores lately? Entry level machines from HP etc. are using integrated graphics to hold prices down yet allow users to beef up their machines when they ahve some coin later on.

    Reply
  • ranger203 - Monday, November 8, 2004 - link

    -1st of all, why does anandtech keep benchmarking AMD FX chips, sure they are the fastest hands down, but none of us are buying they. I.e. they are comparing apples to oranges, (FX vs. P4). They need to bench regular A64s!!!!

    -2nd, Onboard video still really sucks for gaming, but atleast they are making an effort, they should relize that $30 gaming cards are better quality than their onboard video and stop integrating it into their full size atx boards!!! Unless this was just a "show" board of ati's capability, then i could understand....
    Reply

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