32 Bits That Changed Microprocessor Design

In the late 1970s, a time when 8-bit processors were state of the art and CMOS was the underdog of semiconductor technology, engineers at AT&T’s Bell Labs took a bold leap into the future. They made a high-stakes bet to outpace IBM, Intel, andother competitors in chip performance by combining cutting-edge 3.5-micron CMOS fabrication with a novel 32-bit processor architecture.

Although their creation—the Bellmac-32 microprocessor—never achieved the commercial fame of earlier ones such as Intel’s 4004 (released in 1971), its influence has proven far more enduring. Virtually every chip in smartphones, laptops, and tablets today relies on the complementary metal-oxide semiconductor principles that the Bellmac-32 pioneered.

As the 1980s approached, AT&T was grappling with transformation. For decades, the telecom giant—nicknamed “Ma Bell”—had dominated American voice communications, with its Western Electric subsidiary manufacturing nearly every telephone found in U.S. homes and offices. The U.S. federal government was pressing for antitrust-driven divestiture, but AT&T was granted an opening to expand into computing.

With computing firms already entrenched in the market, AT&T couldn’t afford to play catch-up; its strategy was to leap ahead, and the Bellmac-32 was its springboard.

The Bellmac-32 chip series has now been honored with an IEEE Milestone. Dedication ceremonies are slated to be held this year at the Nokia Bell Labs’ campus in Murray Hill, N.J., and at the Computer History Museum in Mountain View, Calif.

A chip like no other

Rather than emulate the industry standard of 8-bit chips, AT&T executives challenged their Bell Labs engineers to deliver something revolutionary: the first commercially viable microprocessor capable of moving 32 bits in one clock cycle. It would require not just a new chip but also an entirely novel architecture—one that could handle telecommunications switching and serve as the backbone for future computing systems.

“We weren’t just building a faster chip,” says Michael Condry, who led the architecture team at Bell Labs’ Holmdel facility in New Jersey. “We were trying to design something that could carry both voice and computation into the future.”

This configuration of the Bellmac-32 microprocessor had an integrated memory management unit optimized for Unix-like operating systems.AT&T Archives and History Center

At the time, CMOS technology was seen as a promising—but risky—alternative to the NMOS and PMOS designs then in use. NMOS chips, which relied solely on N-type transistors, were fast but power-hungry. PMOS chips, which depend on the movement of positively-charged holes, were too slow. CMOS, with its hybrid design, offered the potential for both speed and energy savings. The benefits were so compelling that the industry soon saw that the need for double the number of transistors (NMOS and PMOS for each gate) was worth the tradeoff.

As transistor sizes shrank along with the rapid advancement of semiconductor technology described by Moore’s Law, the cost of doubling up the transistor density soon became manageable and eventually became negligible. But when Bell Labs took its high-stakes gamble, large-scale CMOS fabrication was still unproven and looked to be comparatively costly.

That didn’t deter Bell Labs. By tapping expertise from its campuses in Holmdel and Murray Hill as well as in Naperville, Ill., the company assembled a dream team of semiconductor engineers. The team included Condry; Sung-Mo “Steve” Kang, a rising star in chip design; Victor Huang, another microprocessor chip designer, and dozens of AT&T Bell Labs employees. They set out in 1978 to master a new CMOS process and create a 32-bit microprocessor from scratch.

Designing the architecture

The architecture group led by Condry, an IEEE Life Fellow who would later become Intel’s CTO, focused on building a system that would natively support the Unix operating system and the C programming language. Both were in their infancy but destined for dominance. To cope with the era’s memory limitations—kilobytes were precious—they introduced a complex instruction set that required fewer steps to carry out and could be executed in a single clock cycle.

The engineers also built the chip to support the VersaModule Eurocard (VME) parallel bus, enabling distributed computing so several nodes could handle data processing in parallel. Making the chip VME-enabled also allowed it to be used for real-time control.

The group wrote its own version of Unix, with real-time capabilities to ensure that the new chip design was compatible with industrial automation and similar applications. The Bell Labs engineers also invented domino logic, which ramped up processing speed by reducing delays in complex logic gates.

Additional testing and verification techniques were developed and introduced via the Bellmac-32 Module, a sophisticated multi-chipset verification and testing project led by Huang that allowed the complex chip fabrication to have zero or near-zero errors. This was the first of its kind in VLSI testing. The Bell Labs engineers’ systematic plan for double- and triple-checking their colleagues’ work ultimately made the total design of the multiple chipset family work together seamlessly as a complete microcomputer system.

Then came the hardest part: actually building the chip.

Floor maps and colored pencils

“The technology for layout, testing, and high-yield fabrication just wasn’t there,” recalls Kang, an IEEE Life Fellow who later became president of the Korea Advanced Institute of Science and Technology (KAIST) in Daejeon, South Korea. With no CAD tools available for full-chip verification, Kang says, the team resorted to printing oversize Calcomp plots. The schematics showed how the transistors, circuit lines, and interconnects should be arranged inside the chip to provide the desired outputs. The team assembled them on the floor with adhesive tape to create a massive square map more than 6 meters on a side. Kang and his colleagues traced every circuit by hand with colored pencils, searching for breaks, overlaps, or mishandled interconnects.

Getting it made

Once the physical design was locked in, the team faced another obstacle: manufacturing. The chips were fabricated at a Western Electric facility in Allentown, Pa., but Kang recalls that the yield rates (the percentage of chips on a silicon wafer that meet performance and quality standards) were dismal.

To address that, Kang and his colleagues drove from New Jersey to the plant each day, rolled up their sleeves, and did whatever it took, including sweeping floors and calibrating test equipment, to build camaraderie and instill confidence that the most complicated product the plant workers had ever attempted to produce could indeed be made there.

“We weren’t just building a faster chip. We were trying to design something that could carry both voice and computation into the future.” —Michael Condry, Bellmac-32 architecture team lead

“The team-building worked out well,” Kang says. “After several months, Western Electric was able to produce more than the required number of good chips.”

The first version of the Bellmac-32, which was ready by 1980, fell short of expectations. Instead of hitting a 4-megahertz performance target, it ran at just 2 MHz. The engineers discovered that the state-of-the-art Takeda Riken testing equipment they were using was flawed, with transmission-line effects between the probe and the test head leading to inaccurate measurements, so they worked with a Takeda Riken team to develop correction tables that rectified the measurement errors.

The second generation of Bellmac chips had clock speeds that exceeded 6.2 MHz, sometimes reaching 9. That was blazing fast for its time. The 16-bit Intel 8088 processor inside IBM’s original PC released in 1981 ran at 4.77 MHz.

Why Bellmac-32 didn’t go mainstream

Despite its technical promise, the Bellmac-32 did not find wide commercial use. According to Condry, AT&T’s pivot toward acquiring equipment manufacturer NCR, which it began eyeing in the late 1980s, meant the company chose to back a different line of chips. But by then, the Bellmac-32’s legacy was already growing.

“Before Bellmac-32, NMOS was dominant,” Condry says. “But CMOS changed the market because it was shown to be a more effective implementation in the fab.”

In time, that realization reshaped the semiconductor landscape. CMOS would become the foundation for modern microprocessors, powering the digital revolution in desktops, smartphones, and more.

The audacity of Bell Labs’ bet—to take an untested fabrication process and leapfrog an entire generation of chip architecture—stands as a landmark moment in technological history.

As Kang puts it: “We were on the frontier of what was possible. We didn’t just follow the path—we made a new one.” Huang, an IEEE Life Fellow who later became deputy director of the Institute of Microelectronics, Singapore, adds: “This included not only chip architecture and design, but also large-scale chip verification—with CAD but without today’s digital simulation tools or even breadboarding [which is the standard method for checking whether a circuit design for an electronic system that uses chips works before making permanent connections by soldering the circuit elements together].”

Condry, Kang, and Huang look back fondly on that period and express their admiration for the many AT&T employees whose skill and dedication made the Bellmac-32 chip series possible.

Administered by the IEEE History Center and supported by donors, the Milestone program recognizes outstanding technical developments around the world. The IEEE North Jersey Section sponsored the nomination.