A Technical History of Apple's Operating Systems - Mac OS X Internals

74 Chapter 1 A Technical History of Apple’s Operating Systems
Apple had been 32-bit implementations. The first 64-bit G5 chips—namely, the
PowerPC 970 and 970FX—are very similar to the POWER4 chips since the G5 is
based on the POWER4 architecture. A key difference is that the G5 contains the
VMX 42 vector-processing unit (VPU). Moreover, a single POWER4 chip has two
processor cores, whereas the 970 and the 970FX only have one each. Apple added
the dual-core 970MP processor to its PowerPC offerings in the second half of 2005.
1.5.2.3. Apple Likes RISC: ARM
As Apple was turning to RISC computing in the early 1990s, it collaborated with
other partners besides IBM. In the mid 1980s, the Cambridge-based British com-
pany Acorn Computer Group 43 had developed the world’s first commercial RISC
processor. At that time, ARM stood for “Acorn RISC Machine.” The first version of
the ARM architecture (ARMv1) had 26-bit addressing, with no onboard multiplier
or coprocessor. The first ARMv1 processor, the ARM1, saw limited use in proto-
types of the Archimedes workstation and as a low-cost secondary component in the
BBC microcomputer. It is noteworthy that ARM eschewed some key features of the
prevailing Berkeley RISC architecture: delayed branching, register windows, and
requiring all instructions to execute in a single cycle each.
Even before the AIM alliance was formed, Apple had teamed with Acorn to
fund a new company called Advanced RISC Machines (ARM) Limited. The com-
pany’s goal was to create a new RISC microprocessor standard. VLSI Technology
was an investor and technology partner in this endeavor. It was also ARM’s first
licensee. ARM Limited’s first processor was an embeddable RISC core called the
ARM6. Based on version 3 of the ARM processor architecture (ARMv3), the
ARM6 had full 32-bit code and data addressing. An ARM6 processor—a 20 MHz
42 VMX is the same as AltiVec, which is a Motorola trademark.
43 One of Acorn’s best-known computers was the BBC micro, which, at its launch in 1981, was based
on a 2 MHz 6502 processor.