Power of two textures

Can you explain me, why hardware acceleration required for a long time textures be power of two? For PCs, since GeForce 6 we achieved npot textures with no-mips开发者_运维知识库 and simplified filtering. OpenGL ES 2.0 also supports npot textures without mipmaps and etc. What is the hardware restriction for this? Just simplified arithmetics?

I imagine it has to do with being able to use bitwise shift-left operations, rather than multiplication to convert an (x, y) coordinate to a memory offset from the start of the texture. So yes, simplified arithmetics, from a processor point of view.

I'm guessing that it was to make mipmap generation easier, because it allows you to just average 2x2 pixels into one pixel all the way from NxN down to 1x1.

Now that doesn't matter if you're not using mipmapping, but it's easier to have just one rule, and I think that mipmapping was the more common use case.