Re: Apple II VGA Video Generator Card Project

["Michael J. Mahon" <mjmahon@aol.com>]

Mark McDougall wrote:
> bieling@terra.es wrote:
>
> > But "takes longer" is not a measure of drawing speed. The space, measured
> > in pixels, is missing in this equation : (drawing speed) = (pixels drawn)
> > / (time to draw them).
>
>
> There is another factor to the equation - the memory architecture of the 
> frame buffer. The A2 video memory map is, from a software point of view, 
> quite simply ghastly! The calculations to find a video pixel memory 
> address are orders of magnitude greater than simply shifting an address 
> or even being able to simply increment an address when crossing a byte 
> boundary.
>
> Any 'enhanced' video mode would most likely have a much, much nicer 
> addressing scheme for the frame buffer.

Not really, since the BASIC interface to graphics is all in machine
language.  The built-in routines do not use the obvious, but space-
consuming, acceleration of table lookup for the starting address of
each horizontal line, but most games do.  And horizontal pixel
addressing, though it still requires bit manipulation, can be sped
up significantly by using tables, too.

Further, anything besides 8-bits per pixel is going to require some
bit stuffing, regardless of frame buffer organization.

In most games/animations, most of the drawing time is in 'bitblt',
and speeds are quite directly proportional to the number of bytes
touched.

-michael

Music synthesis for 8-bit Apple II's!
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