Information for Tseng Chipset Users : Mode issues
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## 10. Mode issues

The accelerated driver on ET4000W32/W32i/W32p and ET6000 needs at least 1K bytes of scratch space in video memory. Consequently, if you want acceleration, a 1024x1024 virtual resolution should not be used with a 1Mbyte card. This also means that a 1024x768 mode at 24bpp on a 2.25 MB ET6000 card cannot be accelerated, since you've used up all the memory for the display.

The same thing goes for the ET6000 hardware cursor: it also requires 1kb of free video memory. If that memory is not available, the hardware cursor cannot be used.

The use of a higher dot clock frequencies has a negative effect on the performance of graphics operations on non-et6000 cards (the effect is much less, or even non-existing, on ET6000 cards), especially BitBlt, when little video memory bandwidth is left for drawing. Memory bandwidth is the speed at which data can be pumped into the memory while the RAMDAC is pulling it out to display it on the screen.

Higher dot-clocks (mostly related to higher resolutions) consume more bandwidth, so that less of it is left for drawing into the framebuffer. With a working accelerator, things become increasingly crammed, because modern accelerators can consume huge amounts of bandwidth (but they also give you high speeds in return). High color depths also need extra bandwidth.

If you are short on memory bandwidth (see the separate section on this) and experience blitting slowness or screen "glitches", try using the lowest dot clock that is acceptable; for example, on a 14" or 15" screen 800x600 with high refresh (50 MHz dot clock) is not so bad, with a large virtual screen.

Tseng chips are mostly known for their (very) good memory bandwidth, so you should only start to see problems in the higher regions.

It does not make much sense performance-wise to use the highest clock (85 MHz) for 1024x768 at 76 Hz on a 1 MB ET4000W32; the card will very slow, because there is almost no bandwidth left for drawing. A 75 MHz dot clock results in 70 Hz which should be acceptable. If you have a monitor that supports 1024x768 at 76 Hz with a 85 MHz dot clock, an 1MB card is a poor match anyway.

The ET4000W32i and ET4000W32p have a special feature that almost doubles memory bandwidth (+70%) using "interleaving" between the two banks. Upgrading to 2MB is a real bonus on these cards. This is not true for W32 cards or for ET6000 cards.

Information for Tseng Chipset Users : Mode issues
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Next: Acceleration issues