The companies that dominate the market for microprocessors -- the "brains" of personal computers -- are again under attack. But this time the challenge is coming from computer firms, not other chip makers.

At issue is a streamlined technology for processing data. Known as reduced instruction-set computing, or RISC, it promises to speed up computers, especially technical work stations.

Computer maker Sun Microsystems Inc. of Mountain View, Calif., said it decided to design a RISC microprocessor chip and sell it to other firms only because the established microprocessor firms -- notably Intel Corp. of Santa Clara, Calif., and Motorola Inc. of Schaumburg, Ill. -- were slow to adopt the technology. Some computer makers said that's because the reigning powers don't want to cut sales of their existing microprocessors.

Intel and Motorola deny they have dragged their feet on RISC, but the new chips undeniably will take some of their market. Dataquest Inc. of San Jose, Calif., forecasts that RISC chips will increase their share of the market for 32-bit microprocessors -- the most powerful that are widely available -- from about 9 percent this year to 29 percent by 1992, when advanced microprocessors are expected to be nearly a $1.2 billion business.

Industry observers expect Intel and Motorola to remain the dominant producers of microprocessors, with room for only two or three RISC chips to survive into the 1990s. With a half-dozen RISC chips already on the market and more expected, the result may be a fierce struggle for survival.

"I think RISC is going to be the biggest story in semiconductors for 1988," said Drew Peck, an analyst at Donaldson, Lufkin & Jenrette in New York. "I would go so far as to say that every major semiconductor company has a RISC design completed or under way."

Microprocessors contain instructions that tell them how to handle a computer's operations. Basically, RISC chips include comparatively simple, often-used instructions, but conventional 32-bit microprocessors contain instructions for many exotic computing operations.

Because they carry less baggage, RISC processors can perform on-chip operations many times faster than conventional microprocessors. How much faster depends on, among other things, which instructions are included on the chip and how often the computer needs the other operations.

The field of RISC chips started to get crowded last year with introductions from Advanced Micro Devices Inc. of Sunnyvale, Calif., and Sun Microsystems. Sun and MIPS Computer Systems Inc. of Sunnyvale each lined up three semiconductor companies to make their RISC designs and market them.

Several RISC chips already were available, including the Clipper chip set designed by Fairchild Semiconductor Corp. and now owned by Intergraph Corp., an Alabama work station maker; and chips from VLSI Technology Inc. of San Jose and Inmos Corp. of Colorado Springs, Colo.

So far this year, at least one new RISC design has emerged, from Computer Consoles Inc. of Rochester, N.Y. Motorola plans to announce a RISC processor soon, and Texas Instruments Inc. of Dallas is rumored to be working on one.

Intel's 80486 -- the company's next 32-bit microprocessor, due out in two years -- will incorporate some RISC principles into its design. But Claude Leglise, an Intel marketing manager, said: "I would not call it a RISC processor."

The tug of war over the RISC chip market is reminiscent of the battle among 8-bit microprocessor makers in the 1970s. Experts agree the conclusion may be similar: There will be too many chips available, and few will survive.

"It was just like this in the early days of microprocessors, when everybody and his brother had one," said John East, a senior vice president at AMD. "Three years from now, the astute observer will be able to see who will win this. And in five years it will be down to two or three."

Ten years ago, a dozen companies were vying to sell 8-bit microprocessors, including such companies as aerospace giant Rockwell International Corp. of Pittsburgh and chip maker Zilog Inc. of Campbell, Calif. Today, a handful of companies is on the cutting edge of the business supplying 32-bit microprocessors.

The RISC concept is actually more than a decade old. Scientists at International Business Machines Corp. of Armonk, N.Y., first began measuring the efficiency of instructions programmed into microprocessors in the mid-1970s. They found that only 20 percent of a microprocessor's instructions controlled 80 percent of a computer's processing work. Hewlett-Packard Co. of Palo Alto, Calif., was one of the first to transfer the notions of RISC from the blackboard to the motherboard when it began developing its designs for commercial use five years ago.

It wasn't until recently, though, that system designers found much of a use for RISC. Microprocessor advancements have come largely by improving the amount of information a chip can swallow in one gulp. Early microprocessors could handle instructions four bits at a time. (Bits are tiny electronic pulses that the computer translates into commands.) Next-generation 8-bit chips could digest instructions up to two times as fast. Gradually, the pathways were improved to take on 16 bits, and most recently 32 bits.

Experts say, however, that few pieces of information are wide enough to make it worthwhile to take the next logical jump, to 64 bits. Meanwhile, the appetite for faster engineering work stations led computer makers to search for ways to boost the power of the increasingly popular 32-bit chips.

Although it emerged as a quick fix, RISC is fast becoming a permanent fixture on the computing scene. So far, about a dozen computer makers are selling or plan to sell RISC-based machines.

In addition to Hewlett-Packard -- which based its new Spectrum line of computers on RISC -- companies that sell RISC-based machines include IBM, Silicon Graphics Computer Systems Inc. of Mountain View, Ridge Computers of Santa Clara, Edge Computer Corp. of Minnetonka, Minn., and Prime Computer Inc. of Natick, Mass.

MIPS and Sun, of course, are also on that list. Sun has drawn publicity with its SPARC chip, introduced last June and recently the subject of a technology-sharing agreement with Xerox Corp. and American Telephone & Telegraph Co.

Some experts argue that work station companies might be hesitant to rely on Sun, a competitor, for vital chip designs. But in the meantime, analysts say Sun has used the agreements with Xerox and AT&T -- two large companies without much presence in the work station market -- to boost SPARC's visibility.

''RISC is actually being promoted by the computer companies,'' said AMD's East. ''The thing I like about the Sun chip and the MIPS chip is their violation of the chip market. The czars of microprocessors can't afford RISC, because it's going to be difficult for them to take advantage of it.''

East and other RISC players say Intel and Motorola customers would be forced to choose between their existing chips and their RISC offerings. That would force the chip makers to spend millions to develop a new chip without increasing their microprocessor sales.

Motorola doesn't see it that way; it says many of its customers plan to install its RISC chips in high-end work stations and conventional microprocessors in medium- and low-end models.

And Intel doesn't plan to sell a RISC chip in the computer market because it is committed to the code, or instruction set, contained in its popular 80386 microprocessor and older chips to guide them through their operations.

''Some people say there's a market for RISC and a (conventional) market,'' said Intel marketing manager Leglise. ''I say no. There's a market for existing instruction sets and new instruction sets. What you're not going to see is Intel throw away the existing instruction set.''