History of the ISA: Processors, the PowerPC, and the AIM Triple-Threat

Continuing our series on the Instruction Set Architecture (ISA), this week we delve into the PowerPC ISA.

PowerPC was the outcome of the AIM Alliance founded in 1991: a trio that consisted of unlikely partners, Apple and IBM—who otherwise were competitors—and Motorola, who had a strong relationship with Apple. The three companies developed a family of CPUs based around the PowerPC architecture, in an attempt to create a neutral hardware platform that could be used with a variety of operating systems and applications.

The alliance lasted until about 2005, but PowerPC is still maintained by the POWER Foundation, with a consortium of about 40 companies. Here is a brief history of PowerPC and the AIM Alliance.

IBM 801—Fast Core CPUs

PowerPC’s roots stem from the IBM 801, one of the earliest RISC-based processors that came out of a research project led by John Cocke in the mid-70s. The approach to the IBM 801 was to re-think how processors were designed and focus on improvements in terms of miniaturization and speed. Cocke’s team began with analyzing the trace of programs being run on the IBM System/370 mainframe computer, which could tell them more about where bottlenecks were in processing times.

John Cocke with an IBM 801 prototype. Image courtesy of IBM.

One of the conclusions the team came to was that processors were more complex than they needed to be to accommodate a number of instructions, many of which were rarely used. Part of the IBM 801's optimization was reducing its instruction set to under 100 essential commands, whereas other CPUs like Intel’s 8086 had more than 400. The simplification of the instruction set also allowed the IBM 801 to implement microcode for a broader variety of machines, while also reducing the size of the CPU.

The resulting design would operate at 15 MIPS, ahead of its time for performance and design. IBM would continue on to work on the PowerPC platform based on what the company had learned from the IBM 801.

Apple’s Secret Projects

While IBM was making headway with its new processor design, Apple was becoming concerned with its relationship with Motorola, which had been providing the processors for its Macintosh lineup. The 680x0 processors were no longer keeping up in performance, especially with emerging (and competing) RISC designs.

Porting the Mac OS to a new CPU was not trivial as large parts of it were written in assembly for the Motorola 680x0 processors to make it faster and use less memory. Mac OS also relied heavily on ROMs for the GUI. However, this did not prevent Apple from exploring their options.
First, the company began the Aquarius project, developing a four-core experimental RISC processor. Expert designers, nearly 50 engineers, and a supercomputer were allocated to the project—but, even with this power behind the project, very little progress was made.

In the early 90s, Apple then undertook another project dubbed “Star Trek”, porting the Mac OS to the Intel 80486, a CISC-based processor. Apple was now competing against Windows-based PC computers that were cheaper.

By 1992, Apple gave a demo of Mac OS running natively on an Intel-based PC, showing it could be done.

However, a change of leadership would also change the direction of Apple’s efforts. IBM’s offer to help Apple complete one of its projects, a new OS called Pink, in exchange for using the PowerPC processor in its computers. IBM also made the PowerPC 601 CPU bus to be compatible with the Motorola 88100 RISC processor. This made porting Mac’s OS to the PowerPC CPU easier since less of the operating system’s code had to be re-written, as well as brought in all the benefits of RISC processing.

The Power Apple featured the G5 PowerPC CPU. Image courtesy of Apple.

Motorola was brought on board to manufacture the PowerPC CPUs for Apple—a highly favorable agreement for a company like Motorola that wanted to maintain its relationship with Apple and continue making progress with RISC CPUs.

Thus, the AIM Alliance formally came into existence. Apple’s computers would use PowerPC CPUs for nearly a decade before eventually switching to Intel processors in 2005.

The Fate of PowerPC

While all three AIM Alliance companies were producing systems based around the PowerPC CPU, only Apple’s products experienced success. The group imagined a base in which multiple flavors of operating systems could run on, with high performance, and a competitive price point. Even though the PowerPC CPUs benchmarked quite high, the ultimate downfall in desktop computing would be that many applications still weren’t made available on the PowerPC platform. Eventually, the alliance would break apart when Apple chose to partner with Intel, and Motorola spun off its manufacturing to Freescale Semiconductors.

The PowerPC platform is still in existence today, primarily in embedded systems applications. PowerPC processors are used by Freescale and Xilinx, and video game consoles including the GameCube, Wii, Xbox 360, and Playstation 3. The RAD750, a radiation-hardened PowerPC based CPU, also runs on spacecraft such as the Mars Curiosity Rover and the Juno Spacecraft which studied Jupiter.

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