Apple shares have been rising since Warren Buffett said he's buying them

Apple shares rose about 2 percent in pre-market trading this morning after Warren Buffett's Berkshire Hathaway disclosed an investment stake in the tech giant.

Berkshire's stake in Apple amounts to a little more than 9.8 million shares, worth $888 million as of Friday's closing price. That works out to a little less than 0.2 percent of the 5.48 billion Apple shares outstanding.

Buffett generally isn't known for tech investing and hasn't explained the rationale behind the purchase. He told CNBC the decision to buy the shares was made by one of his lieutenants, Ted Weschler or Todd Combs. They're both former hedge fund managers who Buffett hired in 2010 and 2011.

Buffett has also been involved in the financing efforts behind an effort to buy out Yahoo led by Quicken Loans founder Dan Gilbert. About that effort, Buffett said he's not interested in owning Yahoo, but is willing to help Gilbert finance a bid.

In a filing with the U.S. Securities and Exchange Commission, Berkshire also disclosed that it has boosted its stake in computing giant IBM and the cable TV provider Charter and liquidated its holdings of the wireless company AT&T. Other stakes include American Express and MasterCard.