Downgrades followed semiconductor stocks with exposure to the PC market, including Advanced Micro Devices ( AMD ), Marvell Technology ( MRVL ) and Nvidia ( NVDA ). Sales of new, thin "ultrabook" notebook computers have been soft. And PC makers face an onslaught of low-priced tablets such as Google's Nexus 7, Microsoft's Surface and Amazon's Kindle Fire SD/HD, not to mention the possibility of an iPad mini from Apple. And analysts are beginning to doubt the upside effect from the pending introduction of Microsoft's updated operating platform. "We were hopeful for a boon to PC unit growth from Windows 8; it is increasingly evident to the supply chain that this is unlikely," said Citigroup analyst Glen Yeung, in a Sept. 12 report. Slumping PC orders have left distribution channels stuffed with unsold chips.