WHAT HAPPENED TO CLOCK SPEED

At the start of the PC industry, PC buyers used clock speed to gauge
performance because different PC processors were on par in efficiency.
Part of IBM's decision to use x86 processors for the original
IBM PC was to ensure multiple sources of compatible components.
In ensuring x86 compatibility, processor vendors developed processors
that were similar in their internal designs, or architectures. As a
result, clock speed became the distinguishing characteristic of the
PC processor.
The situation changed, however, in the middle and late 1980s as the
PC began to serve a greater variety of tasks and applications. More
than word processors, PCs became drawing tools, entertainment
appliances, and communication devices. They also began going outside
the office and into homes, on the road, and into the back office
where only mainframes and mini-computers used to reside. From
the increasingly varied uses and locales evolved a greater number
of user profiles, or usage models, with special requirements for PCs
to fulfill.
Accordingly, processor designs evolved, but not merely by scaling
the clock speed. In October 1985, for example, Intel introduced the
80386 processor, which doubled the amount of transistors in the
prior generation's 80286 processor and introduced 32-bit computing
to the PC. These advances were far more beneficial to PC performance
than the advance from the 80286's 12MHz to the 80386's
16MHz. Advances that took place in subsequent designs made the
processor's job easier by integrating small memory caches that
stored the data closer to the processor core. Designers also
improved efficiency by integrating other components, such as float-

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Is Megahertz Enough?
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What does a proper measure of PC performance do? Why
shouldn't PC buyers use processor clock speed exclusively?
IDC believes that a proper measure of performance reflects how
much work a PC does in a given period of time. That requires
knowing how much time the PC worked and how efficiently it
worked. Clock speed alone is not a good measure of PC performance
because it does not account for performance efficiency.
Efficiency is determined by other factors.
Those factors include the processor architecture and how the
rest of the PC — the graphics controller, main memory, hard
drive, and so on — contributes to the work that gets done. Since
a PC has many components and simultaneous tasks, clock
speed only represents one facet of one component and is not
enough to measure the real performance of the entire system.
IDC OPINION
Accordingly, processor designs
evolved, but not merely by scaling the
clock speed.
– 3 – Is Megahertz Enough?
ing point units, and introduced more efficient techniques of data processing,
including adding more processing lines (pipelines) and processing
data and instructions to run the critical tasks first. Designers
also found ways to make processors work better in the context of the
entire system by introducing new instructions (e.g., MMX™ and
3DNow!™ Professional for multimedia) that were optimized for richer
data processing and by giving the processor a faster front-side bus
to the rest of the system.
Due to the different requirements of specific form factors and segments,
different processors from even the same vendor began to
deliver varied performance at the same clock rate. The issue
became more apparent when vendors like AMD moved to develop
their own architectures that, while still compatible with the x86
instruction set, took different approaches to maximize performance.
These approaches included changes that impacted both the processor
and the system (e.g., improved front-side buses).