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More than Clock Speed

As the introduction has already pointed out, many factors influence the performance of a given processor. Table 1 explains these factors in greater detail.

Table 1 – Processor Features and Their Impact on Performance

Feature

Performance Impact

Notes

L2 cache size

Larger cache improves performance

All L2 cache runs at full clock speed in modern processors

L3 cache

L3 cache improves performance when present

The Pentium 4 Extreme Edition is the first desktop processor with onboard L3 cache

Front-side bus speed

Faster FSB improves performance

Fastest FSB speed available with Pentium 4/EE processors is 800MHz; fastest available with Athlon XP is 400MHz

Efficiency per clock cycle

More operations per clock cycle improves performance at a given clock speed and saves power

The Pentium M and the Athlon XP are generally regarded as more efficient processors than the Pentium 4. Consequently, given models of each perform as well as Pentium 4 processors running at faster clock speeds.

Improved branch predictor design

Fewer wasted operations improves performance

Pentium 4 Prescott, Athlon XP, and Pentium M all include various improvements in branch prediction design


With the rise in popularity of laptop and portable computers, other factors such as voltage and the ability to run at reduced clock speeds are important considerations.

Because Intel frequently uses the same brand name (Pentium 4, Pentium M) to refer to processors which might have the same clock speed but differ in other ways, there are several situations in which it's extremely difficult to compare processors. The following sections provide two examples.

Pentium 4 Puzzlements: Prescott Versus Extreme Edition

Intel's latest processors under the Pentium 4 brand name are extremely difficult under the surface. The Extreme Edition, launched in the fall of 2003, is the first Intel desktop processor to feature onboard L3 cache, while Prescott, introduced early in 2004, is the first Pentium 4 processor to use the 90-nanometer (.09 micron) production process. Table 2 shows how these processors compare to each other and to conventional Pentium 4 processors. All of these processors use the 800MHz FSB.

Table 2 – Intel Pentium 4 Extreme Edition and Prescott Comparison

Processor

Clock Speed

L2 Cache Size

L3 Cache Size

Process Technology

Northwood

3.20GHz

512KB

---

.13 micron

Prescott

3.20GHz

1MB

---

.09 micron

Extreme Edition

3.20GHz

512KB

2MB

.13 micron

Northwood

3.20GHz

512KB

---

.13 micron

Prescott

3.40GHz

1MB

---

.09 micron

Extreme Edition

3.40GHz

512KB

2MB

.13 micron


As Table 2 shows, the differences between the Extreme Edition, Prescott and Northwood versions of the Pentium 4 are profound, but how do they translate into real-world performance? Although Intel offers benchmarks which compare Extreme Edition to Northwood processors, it offers no such data for Extreme Edition versus Prescott. To sort out the real-world differences, you'd need to check out third-party tests from sources such as Tom's Hardware, AnandTech and others.

Mobile Muddles: Mobile Pentium 4 Versus Pentium M

Confusion over performance isn't limited to desktop computers. Although the modestly-named Pentium M (the processor used by notebook computers which use Intel's Centrino technology) has a top clock speed of only 2GHz in its latest versions, both the 2GHz and older 1.7GHz versions outperform the Mobile Pentium 4 running at 2.6GHz in both performance and battery life as measured by BAPCo's MobileMark 2002 benchmark http://developer.intel.com/products/benchmarks/notebook/index.htm

Table 3 compares the major features of the processors tested.

Table 3 – Intel Mobile Processors Compared

Processor

Clock Speed

L2 Cache Size

FSB Speed

Process Technology

Pentium M

1.70GHz

1MB

400MHz

.13 micron

Pentium M Processor 755

2.0GHz

2MB

400MHz

.09 micron

Mobile Pentium 4

2.6GHz

512KB

533MHz

.13 micron


As Table 3 makes clear, buying the "fastest" processor doesn't mean you'll have the 'fastest' laptop computer.

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