The New York Stock Exchange said a four-hour outage in its trading systems Wednesday was caused by problems with a software upgrade.
Walgreens Boots Alliance jumped 4 percent.
The Dow Jones industrial average rose 33 points, or 0.2 percent, to 17,548. The Dow slumped 261 points the day before.
The Standard & Poor's 500 increased four points, or 0.2 percent, to 2,051 and the Nasdaq gained 12 points, or 0.3 percent, to 4,922.
Bond prices fell. The yield on the 10-year Treasury note rose to 2.30 percent.