Mode issues

Information for Cirrus Chipset Users : Mode issues
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## 4. Mode issues

The accelerated 256-color driver uses 16K bytes of scratch space in video memory, and the hardware cursor also uses 1K (2K on the '6X). Consequently, a 1024x1024 virtual resolution should not be used with a 1Mbyte card.

On older chips, the use of a higher dot clock frequencies has a negative effect on the performance of graphics operations, especially BitBlt, when little video memory bandwidth is left for drawing (the amount is displayed during start-up for 542x/3x/46 chips). For the 542x/3x chips, with default MCLK setting (0x1c) and a 32-bit memory interface, performance with a 65 MHz dot clock can be half of that with a dot clock of 25 MHz. So if you are short on memory bandwidth and experience blitting slowness, 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.

5434-based cards with 2Mbyte of memory do much better at high dot clocks; the DRAM bandwidth is basically double that of the 542x series. The 543x chips also make more efficient use of the available DRAM bandwidth. The same goes for the 544x.

Information for Cirrus Chipset Users : Mode issues
Previous: XF86Config options
Next: Linear addressing and 16bpp/24bpp/32bpp modes