A Motorola unit laid off 500 employees.

The Four-Phase Systems unit of Motorola announced 250 layoffs at its Silicon Valley headquarters and another 250 outside the Bay Area. Most of the cutbacks, blamed on a continuing slump in the computer business, become effective today and will leave the firm with 1,950 employees in Cupertino, Calif., and about 3,800 around the world. The company, which makes microcomputers and minicomputers, began streamlining its work force and restructuring operations last fall.