Mode issues

# 4 Mode issues

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

The use of a higher dot clock frequencies has a negative effect on the performance of graphics operations, especially BitBlt, when little DRAM bandwidth is left for drawing (the amount is displayed during start-up). 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 DRAM 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.

It does not make much sense performance-wise to use the highest clock (85 MHz) for 1024x768 at 76 Hz on a 542x; the card will almost come to a standstill. 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, a standard 5426/5428 based card is a poor match anyway.

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.

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