Originally Posted by sunilraman
Er, may I interject, I get what you're saying, but there's no 4x as much memory, it doesn't scale that way. The only 4x we know for sure is in the final output to the framebuffer, ie, 12MB required instead of 3MB. But everything up to that point does not necessarily mean 4x the memory required.
I do still however think something over 500MB is the minimum RAM, with 1GB being just nice. It's hard to imagine the GPU not needing less than 100MB (but not directly 4x the RAM that the iPad 2 GPU uses)
The compositing system in use on iOS, Mac OS X, etc, doesn't construct the framebuffer each frame from nothing, like a Mac OS 1-9 application. If it did then we would be dealing with 12MB * 2 only, plus application graphics data.
However, each app has its own render buffer. This may be double buffered for smoothness. That includes the dashboard and potentially the lock screen.
The framebuffer is composited every frame from an OpenGL context describing the exact display at that time - for example you may be scrolling between screens on the dashboard - I expect that's an OpenGL effect that ties together two screens (using the iOS equivalent of Core Animation). 24MB double buffered = 48MB to scroll between iOS home pages, and on top of that will be the icons and text and other stuff to be rendered. Very little CPU work, and the GPU can do it in its sleep too, but it uses RAM.
Of the current 512MB in the iPad 2, we can assume that ~256MB will be for graphics use only (media, screens, etc). 4x that is 1024MB, plus the 256MB that Apps, OS, etc can use for executable code and their data structures. 1.25GB before considering the need for apps to have more memory for their own datastructures (each tab in a browser can use tens of MB) as time moves on. You can move the graphics slider down - 160MB? still 992MB required with a larger display. Many people would argue to move the graphics slider up, but they would have to argue for a 2GB iPad 3!
Of course disabled apps can be removed from memory (take a snapshot of the UI for when it is restored) so maybe it isn't all that bad. But we don't want apps getting removed from memory too eagerly.