AT&T has introduced an Intel 80286-based personal microcomputer that runs MS-DOS and Unix System V operating systems concurrently. AT&T may also introduce a replacement board for its PC 6300 that will enable the year-old machine to run both MS-DOS and Microsoft Corp.'s Xenix concurrently. Then the PC 6300 could run the same Xenix programs as the IBM PC/AT. The moves are designed to take advantage of the increasingly popular UNIX operating system and to make AT&T micros capable of competing head-to-head with IBM.