U.S. stocks broke into the red by mid-afternoon after a choppy morning spent riding the flatline. Investors sat on the sidelines ahead of a closely-watched speech from Federal Reserve Chair Yellen on Friday. Micron Technology (MU) - Get Report climbed after HP (HPQ) - Get Report execs forecast a shortage in DRAM and flash memory. Analysts at Evercore and Nomura expect the shortage to benefit demand for the chipmaker's products. Workday (WDAY) - Get Report rose thanks to better-than-expected sales in its recent quarter. William Blair analysts note that acceleration in its subscription billings over the second quarter eased many investors' concerns. St. Jude Medical (STJ) slid after Carson Block's research firm Muddy Waters said it is short the stock. The firm expects the company to lose more than half its revenue for at least two years due to device recalls. St. Jude called the allegations 'absolutely untrue.'