Phones work on a push model of service and thus have a light os. It does something normally until it receives a push update, such as receiving a call, a text message, a sms, etc. It does not have to actively dedicate resources waiting and scanning for something it only uses the resources until they are required to do so.

Furthermore while the OS may do multiple threads at once, the User doesn't have true multitasking with the iphone. These are the the things that can still occur when you switch apps with an iphone/ipad.

- Background audio
- VoIP
- Background location
- Push notifications
- Local notifications
- Task finishing
- Fast app switching

So you can still receive audio from Pandora while you use another app, or you can access the voip function, or you can recieve push notifications from apples servers. All other things fall under the Fast App Switching when you do this, the old app pauses exactly where it was while you switch to the new app. This is unlike windows or mac where those programs can run concurrently.

Thus I don't believe the overhead of the phone OS is going to be significantly more than the 3ds, it will be more for sure but not significantly more. Even if it is more the overhead will only be felt on the cpu and the memory and not the gpu.

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The cpu chips in iphones and ipads are much faster than the chips in nintendo 3ds. Arm11 is just plain old. The Iphone 4 and the Ipad uses a chip called the A4 which is a cortex a8 at 1 ghz. At the same mhz the a8 is about 66% faster than an arm11 cpu, that makes a single a4 about 3 times faster than the current dual core arm 11 cpu the 3ds uses.

The Ipad2 furthers the gap, it uses a chip called the A5 which is a dual core cortex a9 at 1 ghz. A single cortex a9 is about 25% faster than a cortex a8 at the same clock speeds, thus 108% than an arm11 at the same clock speed and the same number of cores. This makes the A5 over 7 times faster than the cpus in the nintendo 3ds (remember there are two cpus in the A5 not one).

Overhead is not going to eat 3 to 7 times more cpu horsepower.

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The ipad 1 has 256mb of memory, the ipad 2 and iphone 4 has 512mb of memory.

The nintendo 3ds has 128mb of memory.