which feature are you going to remove or reduce to have it?
None.
That's the point.
The architecture of both ps3 and xb360 is scalable.
Cost of BC? Zero.
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In the case of ps3, Sony literally had to have the ps2 chipset
in addition to the ps3 chipset. This added cost did nothing for ps3 games as the architecture was so different to ps3 that the hardware (and cost) was there solely for the purpose of playing old ps2 games.
Xb360 had a purely software solution which did incur one cost element on top of the engineering cost which was licensing the nvidia gpu tech for emulation.
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Contrast this with xb720/ps4 potential designs:
Take the Cell, put 4 of them together, viola, ps4 cpu. Then, grab whatever gpu they like and minimal effort will be needed for emulating RSX.
Take the xenon, put 4 of them together for a total of 12 cores and viola, xb720 cpu. The grab the latest ATI GPU which should have minimal problem emulating ATI's prior chip.
Both efforts are low cost as both companies already own the designs of the cpus.
Nextgen the GPU will be leaned on more for gpgpu calculations etc anyway so the cpu "power" is not a major concern.
The above examples are simplified, but essentially, doing just this simple scaling, would result in suitable CPU's for nextgen. Anything more as far as customizations would be icing on the cake.
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As for whether or not my xb360 can play SNES carts, no ... but I can sell, trade, lend, lease, store, or play them whenever I please. One more thing, you don't see SNES carts topping the charts much these days either ... Nor would I expect it in the future...
I fully realize the DLC model, but there is an assumed responsibility with DLC by the platform holder to "take care" of the digital media which was purchased.
And as I said above, as much as they may WANT to, they will have a very difficult time in selling all the same old digital remakes (or HD versions) AGAIN to a userbase that was just screwed out of their prior purchases.
Best case scenario for them is less purchases by the same number of people. Worst case scenario is less purchases
by less people.
And what would this sacrifice be for?