US stocks have fallen further after Donald Trump imposed a new trade restriction on the chip designer Nvidia, rattling investors and triggering a sell-off across the semiconductor industry.The S&P 500 index dropped by about 1.3% in early trading, with the tech-heavy Nasdaq index down 2.1%. The Dow Jones fell 0.6%.Nvidia, the Californian company at the heart of the revolution in artificial intelligence technology, lost billions of dollars from its market value at the opening bell, with its shares down about ...