National Semiconductor Corp. plans today to announce that it is able to combine the functions of most of the chips used in personal computers onto a single chip, which could bring PC prices under $500 and lead to a host of new computing devices. National, the country's fourth-largest chip maker, said the device will replace a dozen or more separate chips typically found in PCs and combine technologies it has developed and purchased in recent years. In November, Santa Clara, Calif.-based National completed a $500-million merger with Cyrix Corp., a maker of Intel Corp.-compatible microprocessors, giving it an arsenal of products to create a PC with one chip, excluding system memory. National Chief Executive Brian Halla will discuss plans for the new chip at a semiconductor industry conference in Phoenix today. He said the company will have the first working version of its chip by year-end and can produce it in volume by June 1999. The company believes the new chip will lead to even lower-cost PCs and other "information appliances." Halla predicted PC prices could fall to $400 to $500 with National's new chips.