The industry's leading barometer, the book-to-bill ratio, remained at a high level for the three months ended in April, the Semiconductor Industry Assn. said. The ratio was 1.20 in April. (A ratio of 1.20 means that for every $100 of products shipped, computer chip makers received an average $120 worth of new orders.) The ratio for March was revised downward to 1.20 from 1.21 reported last month. The March and April level of 1.20 is the highest since 1984.