Shares of Advanced Micro Devices posted their worst day since July on Thursday as the popular semiconductor stock appeared to take a breather.

AMD's stock sank 5.3 percent by the closing bell, finishing the day at $30.48, well below recent 12-year highs.

The shares, which opened the day higher, rose more than 7 percent in the previous session amid Apple's latest iPhone unveil, an important annual event for Wall Street's top chipmakers.