Microsoft’s displacement by Apple as the largest technology firm (as measured by market cap) on Wednesday lead to an upgrade for the company’s stock by Goldman Sachs analyst Sarah Friar. In a research note to clients covered by Bloomberg, Ms. Friar said that MSFT now has an attractive valuation, and she issued a “Buy” rating on the stock.

“While European contagion is a concern, Ballmer’s comments were not Microsoft specific,” Ms. Friar wrote. “The shares underperformed large-cap tech despite having relatively lower exposure outside of the U.S. Given significant underperformance and attractive valuation levels, we are buyers of the shares.”

Mr. Ballmer had made comments about economic concerns in Europe, saying that the underlying problems with the EU economy would spread. That helped send the company’s stock lower, but Ms. Friar’s research note was intended to tell investors that Mr. Ballmer’s comments shouldn’t be interpreted as meaning he expects his company to suffer specific negative effects from said troubles.

As we reported earlier Thursday, a combination of recent gains by AAPL and the above-mentioned losses by MSFT resulted in Apple having a larger market cap than its long-time (erstwhile?) nemesis. Market cap, which is short for market capitalization, is a measure of a company’s value as expressed by the number of outstanding shares multiplied by the current share price.

While it may seem like an arbitrary number that overly discounts revenue and earnings — Microsoft is larger than Apple in both of those metrics — market cap is often a direct expression of how Wall Street and the market as a whole values the future prospects for a company.

As of this writing, AAPL’s market cap is US$228.6 billion (909.9 million shares outstanding x $251.24 per share). Microsoft’s market cap is $227 billion (8.8 billion shares outstanding x $25.90 per share).

Both shares were showing gains in Thursday’s trading session, with MSFT trading higher after Ms. Friar’s upgrade at $25.90, a gain 0.89 (+3.56%), on strong volume that already surpassed the stock’s normal daily average of 82.4 million shares.

AAPL is also showing a smaller gain in percentage terms, with the stock trading at $251.24, up $7.131 (+2.92%), on what is shaping up to be light volume.

*In the interest of full disclosure, the author holds a small share in AAPL stock that was not an influence in the creation of this article.