This is a very informative excerpt from the MacNN Forums about the Intel GMA 950 video card found in all Apple MacBooks. MacBook users, read this!
“Put simply, there are two types of “things” that go into video memory:
1) The frame buffers
2) Textures/Surfaces
The GMA950 has a maximum resolution of 2048×2048 pixels with 32bpp color depth. That means a single frame buffer can take up to 16MB.
It can also display two unique images simultaneously. In the case of the Macbook, this means the internal LCD panel and the external DVI connector. Taking this into account we’re up to 32MB — in practice this will never happen since there really aren’t any displays with that high a resolution on the market. However, Apple could be playing it safe and always reserve enough memory to handle all frame buffer sizes.
Similarly, Apple may be using the concept of having front and back buffers to provide smooth video. This means each display effectively has two frame buffers associated with it. As a result, now we’re up to the magic 64MB number.
In addition to frame buffers, there are typically off-screen areas that are used to cache bitmaps, individual characters of fonts, the contents of a window that is going to be blitted or blended, etc.
There are countless uses for off-screen memory. Importantly though, the GMA950 can only off load work from the CPU when it has this memory to use. It can only accelerate drawing and compositing within video memory.
In this case, it sounds like Apple has ensured that there is a minimum of 16MB of off-screen memory for this use. As a result, we get the 80MB number.
The important thing to realize about integrated graphics chips such as the GMA950 is that memory can be dynamically moved from the system to the video processor at will. This is done in 4KB chunks up to the maximum address space of 256MB.
We already know from IOReg that Apple has configured the GMA950 with a 256MB aperture.
And Apple themselves have effectively confirmed all of this with their knowledge base article:
Mac mini (Early 2006), MacBook, iMac (Mid 2006): About the Graphics Processor
In short, the 64MB number being thrown around is a minimum value not a maximum value.”
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