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you always read about shaders not working on cards that support the given shader number
Its a bit more complicated than this, but since shader model 2.0 such compatibility problems have vanished.
The problem was that the early pixel shader versions were not really programmable..they relied more on certain hardware routines Also drivers made a big difference.
GeForce FX cards, while they supported ps 2.0, were designed so poorly that standard 2.0 code couldnt run at a reasonable speed, so a new version, 2.0b had to be introduced...
Since radeon 9700 and GeForce 6 series and better, all these issues have been addresed. Now there are still some differences in card implementations, but are far more subtle...mostly they will not affect the user..
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