A new company has introduced a product for minicomputers -- those now-inaptly named machines that cost hundreds of thousands of dollars -- to make them perform like inexpensive desktop models.

"Perhaps it seems odd to go to a lot of effort to make a big machine act like a small one," says Garrett Gruener, president of Virtual Microsystems Inc. of Berkeley, Calif. But what makes the effort worthwhile, he explains, is software. The burgeoning ranks of microcomputer (personal computer) owners have created a vast new market for off-the-shelf programs, resulting in a wider, less-expensive selection of software than that enjoyed by minicomputer users.

The inspiration for the product, called "The Bridge," came when Gruener, formerly marketing director for a computer graphics and robotics company, was shopping for a project-scheduling program to use on the company's mini. All available prodncts carried price tags higher than $15,000 He didn't think they were worth it.

Then he came across a $295 personal computer program that could do ihe job. So he asked a computer-consultant friend to design software that would allow it to be used on the minicomputer. When they realized that others might be encountering similar situations, the new company was born.

Using The Bridge to make microcomputer programs work on minis carries a number of advantages over using the same programs on personal computers. For example, users don't need expensive floppy disk drives; they simply save their work on the mini's large, hard disk drives. And they can take advantage of other typical minicomputer peripherals, such as high-speed printers. Any number of users can run different microcomputer programs from the mini's terminals simultaneously. The Bridge, which sells for $3,500, allows use of any software written for "CP/M," the popular microcomputer operating system.

One disadvantage seems to be that programs operate somewhat more slowly on The Bridge than they would on a genuine micro. But Virtual Microsystems has introduced "z-Board," a $4,000 hardware accessory for The Bridge that will allow it to operate at the same speed as a stand-alone micro.

One of Virtual Microsystems's first customers was Diasonics Inc., a manufacturer of medical imaging systems in Milpitas, Calif. Says Roy Lum, Diasonics's manager of management information systems: "Even though we have terminals all over the company where people can access our minicomputers, half a dozen employees bought personal computers at an average cost to the company of $6,000 each. Now that we have The Bridge, I'm not allowing anyone else to buy micros."