The Sensex at the Bombay Stock Exchange gained 231 points, or 1.2 percent, to close at a six-week high of 20,256, on Wednesday, ahead of the monthly expiry in the derivatives segment, and amid rising Asian markets. The Nifty at the National Stock Exchange jumped 64 points, or 1.1 percent, to 6,060.Stock markets in Japan, China, Hong Kong, South Korea and Singapore rose between 0.5 percent and 1.5 percent.