Out of the total volume of A8 chips, Samsung is producing around 30 percent, while TSMC is making 70 percent,. It is preferable in the electronic industry to have more than one vendor producing the semiconductor chips. If you have multiple sources for your chips you are not too dependent on only one supplier for your products. You also have more flexibility in price negotiations. It is interesting that earlier iPhone were dependent on only Samsung producing the microprocessor running the cell phone.
However, since fab lines are very expensive (see below and at Samsung To Invest Nearly $15B In New Chip Plant ) often vendor are stuck using only one foundry source. Different foundries processes are not identical and it will take long time to copy chip design from one vendor to another.
For the next generation of iPhone it appears that Samsung already has a contract in place with Apple to produce the A8's successor, tentatively named the A9, which will be made using the 14-nanometre process.
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