Apple Inc. has become the world\'s most valuable company as its market value passed the rarefied half-trillion-dollar mark in trading Wednesday, climbing to a mountain peak where few companies have ventured, according to media reports. Apple\'s stock jumped 7.03 dollars, or 1.3 percent, to 542.44 dollars on Wednesday, putting the Cupertino, Calif., company\'s value at 506 billion dollars when regular trading ended. Apple was running neck and neck with Exxon Mobil Corp., which is now worth about 407 billion dollars, for the title of most valuable firm for much of last year. Only a few companies have reached the 500-billion-dollar mark, thotgh none managed to stay there for long. According to Los Angeles Times report, Exxon stayed at about 500 billion dollars for two brief periods toward the end of 2007 and Microsoft Corp., , now worthing about 267 billion dollars, passed the 500-billion-dollar line in 1999 and then again in 2000. Also passing 500 billion dollars a decade ago were Cisco Systems Inc., Intel Corp. and General Electric Co., the reports said. Apple stock has risen steadily over the past few years on the release of a pathbreaking trio of mobile devices starting with the iPod in 2001, followed by the iPhone in 2007 and the iPad in 2010. Apple sent out invitations Tuesday to a March 7 press event expected to be the unveiling of the next generation of the iPad.