Mail Archives: djgpp/1997/07/01/13:21:20
Christian Granström writes:
>I have been coding demos on the Amiga since the late 80:s.
>To avoid flicker on the Amiga you just keep two buffers
>in your videomemory. Then you switch between those during
>a vertical blanking.
>
>On the pc this seems to be a lot more complicated. The method
>I used on the Amiga could be compared to banking or...?
No. Banking is the term used to describe the method used to access video
memory before linear framebuffers came along, where there was only a 64k
range of memory addresses (at 0xA0000) mapped to the video card. Since
SVGA modes use more than 64k of image data, that 64k aperture was
remapped to refer to different areas of the real, larger block of video
memory, by setting hardware registers in the graphics card. So first
you'd set the bank indicating which 64k block you wanted to access, and
then you would refer to the appropriate offset within that 64k block.
Slow and clunky, but even today there are some cards that don't support
linear addressing so it's still a good idea to support this method of
access...
>But using a Linear Framebuffer is much easier. But then you
>have to move all data from a buffer into the videomemory wasting
>a lot of cpu power in larger resolutions.
Not necessarily: you could draw the whole thing directly into video ram,
and flip between two pages like you would on the Amiga (assuming you
have enough video ram, of course: you need a 1 meg card to hold two
640x480 pages). The reason many people prepare the images in main memory
is that it's harder to access video RAM (memory protection, and
potentially the bank switching and 64k apertures can be a pain), and
also that video memory is a lot slower to access than regular RAM. If
you are going to be overdrawing the same pixel many times (eg. plotting
a background tile, overlaying it with a sprite, and then maybe drawing
some parallax stuff or other sprites over the top of that), it can often
be faster to do this in main memory and then only copy the finished
image (one write per pixel) to video memory, rather than doing the
drawing straight to the screen. Which is best depends on exactly what
you are doing, and the speed of the bus and graphics card in your
machine...
--
Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
Beauty is a French phonetic corruption of a short cloth neck ornament.
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