The fate of the S&P 500 index ' used by investors as a barometer for the health of corporate America, and cited by presidents as a measure of their handling of the economy ' often comes down to just two companies: Apple and Microsoft.

Even after the failures of two regional banks in the United States and the rescue of a global investment bank in Europe sent a jolt through the financial system and raised new fears about an already fragile global economy, the S&P 500 ended the month up 3.5 percent.

Apple and Microsoft accounted for about half of that gain, according to data from S&P. Both were seemingly immune to the banking crisis and boosted by fervor over artificial intelligence, with Apple rising 11.4 percent during the month and Microsoft 15.6 percent.

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