Computer breakdowns shook American equity markets again today as malfunctioning software that feeds data between exchanges prompted Nasdaq Stock Market to halt trading in thousands of stocks and options.

Nasdaq said trading in shares it lists had been stopped amid issues at its Securities Information Processor, the feed that disseminates quotes and prices. The second-biggest stock market operator in the U.S. halted transactions in what it calls Tape C, which comprises all Nasdaq-listed securities.

Buying and selling in many of the country's most heavily traded shares, from Apple Inc. to Intel Corp. and Facebook Inc., ground to a virtual halt as brokers were unable to execute customer orders. The Nasdaq 100 equity index stopped moving shortly after noon, according to data compiled by Bloomberg. The Dow Jones Industrial Average, which has Microsoft Corp. and Cisco Systems Inc., continued to update.