ET6000 memory bandwidth hype and the impact on video modes

Information for Tseng Chipset Users : ET6000 memory bandwidth hype and the impact on video modes
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## 11. ET6000 memory bandwidth hype and the impact on video modes

Tseng has always had wet dreams about memory bandwidth, and their press announcements about the ET6000 memory bandwidth are no exception.

They claim the ET6000 using MDRAM is capable of reaching an incredible 1.2 Gbytes/sec of bandwidth. That would surpass just about everything on the market (even SGI).

And that would be true, _if_ they actually used the fastest available MDRAMs on their boards, which they don't. The stunning 1.2 GByte mark is only reached when using 4 MDRAM chips at their max clock rate of 166 MHz. But due to design limitations, the first-generation ET6000 can only drive the memories at 92 MHz (that will change when the ET6300 hits the streets).

This means the max. theoretical bandwidth available on current ET6000 boards is "only" 360 MB/sec on boards with 2 MDRAM chips, and 720 MB/sec on boards with 4 MDRAM chips. And this assumes a best-case situation (=extremely long bursts -- the MDRAMs use a shared address/data bus, much like the PCI bus does). In the real world, unaligned accesses both from the PCI bus and the accelerator will reduce the effective available bandwidth. The current ET6000 boards peak out at about 225 MB/sec, with 2 or 4 MDRAMs.

Memory bandwidth limits the maximum resolution you can use at a given color depth. The ET6000 RAMDAC can cope with 135 MHz in any situation. But the RAM cannot. At 32bpp (sparse 16M color mode), using a 135 MHz pixel clock would require a memory bandwidth of 135*4 = 540 MB/sec, which the current ET6000 boards simply cannot cope with. And then you still need some spare bandwidth for the PCI bus and the accelerator.

That is why some modes will be refused, depending on your MDRAM memory layout, even if the amount of memory would permit such a mode. See also the trouble shooting section to see what can happen if too little memory bandwidth is available.

Information for Tseng Chipset Users : ET6000 memory bandwidth hype and the impact on video modes
Previous: ET6000 memory size facts and fiction
Next: Linear addressing and 16bpp/24bpp/32bpp modes