only ran at 133MHz anyway. More to the point, RDRAM was
much more expensive than SDRAM – around three times
the price, in fact.
Intel had another option, the cheaper 810 chipset, which
supported 133MHz SDRAM, but again had a crucial flaw
- there was no AGP slot, so you couldn’t run the latest
graphics cards on it. If you were a gamer, you had to buy
into 820 and, needless to say, we were more inclined to
buy a cheaper SDRAM-based Athlon system instead, or
to overclock an Intel 440BX motherboard from the last
generation. Intel later addressed this with the SDRAM and
AGP-supporting 815 chipset, but by this time it was too late
to stop the Athlon onslaught.
Intel needed an answer, and it had one in the works – a
brand-new CPU microarchitecture that rewrote many of
the previous rules and could be clocked to high heaven.
The Pentium 4 would be built on several new technologies
and principles, and leave existing CPUs looking like relics of
yesteryear. At least, that was the idea.
NETBURST FORTH
Intel called its new microarchitecture NetBurst, and it
represented a very different approach to the Pentium II and
III, which had been largely based on the earlier Pentium
Pro’s P6 core. At this time, all CPUs had just the one core,
and clock speed was the primary indicator of a CPU’s
performance. There were no model numbers like we have
today – you bought a 1GHz Pentium III, for example, or an
800MHz Athlon.
This is where NetBurst could beat previous CPU
microarchitectures, as it was built to be scaled up to super-
high clock speeds. In fact, at the Pentium 4 launch in 2000,
Intel said it expected the architecture to scale up to 10GHz
as fabrication processes were refined over the years.
Yes, really.
That didn’t happen, as we now know, but NetBurst
could hit very high clock speeds for the time, despite the
first ‘Willamette’ chips being built on the same 180nm
process as the previous ‘Coppermine’ Pentium III chips.
They launched at 1.4GHz and 1.5GHz, with a 1.7GHz model
arriving a few months later and a 2GHz model coming out
in the summer of 2001.STUCK IN THE PIPELINE
That sounds amazing, you might think. Intel had gone
from 1GHz to 2GHz in under two years, and its new
microarchitecture was clearly built for high-frequency
operation. However, achieving that high clock speed
required some fundamental changes to the structure of
the microarchitecture, one of which was a large increase in
the number of stages to the execution pipeline – with the
pipeline split over more stages, Intel could devote fewer
transistors to each specific stage, enabling it to increase the
clock speed.
The first Willamette Pentium 4 CPUs had 20 pipeline
stages, which increased to a massive 31 stages in the
later Pentium 4 CPUs, codenamed Prescott. As a point of
comparison, the Pentium III had 14 pipeline stages, the first
Athlon 64 CPUs had 12 pipeline stages – even Intel’s latest
Comet Lake cores have fewer than 20 stages. NetBurst had
a long pipeline, especially for the time.
There are two main problems with a long pipeline – the
first is that they require a higher voltage than a shorter
pipeline, and therefore the CPU generates more heat,
especially in conjunction with the high clock speeds.A die shot of a
Prescott Pentium 4,
which had a colossal
31-stage pipeline