Mail Archives: djgpp/1998/10/29/06:31:32
Ludvig Larsson writes:
> I downloaded a vbe.zip example and got my lfb running,
> but first, in my 640x480x16bit mode, with doublebuffer,
> actually, the bankswitches doesn't eat about any time
> (well, sure they do, but the time it takes to perform
> these 9-10 switches is really non-performance related)
Absolutely. If you are just blitting an image from memory to the
screen once per frame, that only needs a small number of bank switches
and so this issue is not really critical. It is when you start drawing
directly to video memory, which can often require several thousand bank
switches per frame, that these things become important.
> Well, the lfb then, is super slow...
Even slower than your bank switching code? I find that hard to
believe...
> I have an old Matrox Millennium(well, two years old pci etc)
> and, sure, maybe I have made some stupidities in the lfb
> code, but what can one expect as time to make a bankswitch
> on a "normal" card?
It is extremely variable depending on your VESA driver. The mode switch
is always quite expensive, but some drivers will be much faster than
others when it comes to actually doing the switch.
For a simple blit, on most hardware you will see very little difference
between LFB and banked modes. On some recent cards, though, the banked
modes are only emulated for backward compatibility, and using these will
force the card to use a slower bus access method. On such hardware,
using LFB modes will allow much faster data transfers in addition to
removing the bank switches themselves.
Shawn Hargreaves.
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