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<A HREF="cirrus.html"><EM> Information for Cirrus Chipset Users</EM></A>
 <b>:</b> <EM>Mode issues </EM><BR>
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  <H2><A NAME="4"></A>4. Mode issues </H2>

<p>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.</p>
<p>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/46/6x 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.</p>
<p>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.</p>

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<A HREF="cirrus.html"><EM> Information for Cirrus Chipset Users</EM></A>
 <b>:</b> <EM>Mode issues </EM><BR>
<b>Previous:</b> <A HREF="cirrus3.html"><EM>XF86Config options </EM></A><BR>
<b>Next:</b> <A HREF="cirrus5.html"><EM>Linear addressing and 16bpp/24bpp/32bpp modes </EM></A>



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