International Business Machines yesterday mounted a direct attack on two of its biggest rivals, Hewlett-Packard and Sun Microsystems, with the introduction of powerful new Unix computers and updated software.

These are aimed squarely at the market segments dominated by the two American west coast computer-makers.

IBM has competed in the Unix server market for many years, but has placed more emphasis on its mainframe computers, that run proprietary software, and lower-performance servers running either Windows NT or other operating systems.

Yesterday, the company unveiled a machine that it claims is the world's most powerful Unix server. The new S80 server, which comes with up to 24 microprocessors, more than triples the performance of IBM's current top-of-the-line Unix server and outpaces Sun's fastest machines, IBM claims.

IBM also introduced a new line of computers aimed at Internet service providers and application service providers.

IBM also upgraded its version of Unix, called AIX, and laid out plans for future hardware and software upgrades.