The Sensex at the Bombay Stock Exchange fell 75 points, to 18,134 in opening trade, this morning, on continued selling by investors, amid weak global markets. Later, the Sensex swung between negative and positive territory, in very volatile trade, and stood a small 13 points, or 0.1 percent in the positive zone, at 18,223 in afternoon deals, a short while ago. The 30-share Sensex has already lost more than 660 points in the past three trading sessions.Other Asian markets in South Korea, Singapore, Indonesia, Hong Kong, Japan, and Taiwan were down by between 0.1 percent and 1.4 percent, today, as the US debt impasse kept market sentiment subdued. Over in the US, the Dow Jones Industrial Average had lost 0.5 per cent, overnight.