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 6%. The sell-off, which has spread to semiconductor makers in Asia and Europe, comes after Nvidia said the…