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Central processing units are the brains behind any computer system.These series of pages provide specific information about the specifications behind various Intel and AMD based processors.
Complete guide to Central Processing Units
Introductions Principles Of The CPU Evolution Of The CPU Overclocking
CPU Cooling Chipsets and Bus Types CPU Connectors Conclusions
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Quick Specifications:
Processor Label: Intel P6 (686) # Of Transistors: 5.5 million
Generation: 6th Generation Cache Memory: 256KB L2
Release Year: From mid-1997 to early 1999 Memory Bus Speed: 66 mhz to 100mhz
Data Bus Width: 32-bit Clock Speed Range: 233 MHz to 450 MHz
Address Bus Width: 32-bit    
More Information:
Picture of this processor.

Intel P6 (686) Processors

The P6 (686) processors represent a new generation with features not found in the previous generation units. The P6 processor family began when the Pentium Pro was released in November 1995. Since then, Intel has released many other P6 chips, all using the same basic P6 core processor as the Pentium Pro. Table 3.13 shows the variations in the P6 family of processors.

Table 3.13. Intel P6 Processor Variations
Pentium Pro Original P6 processor, includes 256KB, 512KB, or 1MB of full-core speed L2 cache
Pentium II P6 with 512KB of half-core speed L2 cache
Pentium II Xeon P6 with 512KB, 1MB, or 2MB of full-core speed L2 cache
Celeron P6 with no L2 cache
Celeron-A P6 with 128KB of on-die full-core speed L2 cache
Pentium III P6 with SSE (MMX2), 512KB of half-core speed L2 cache
Pentium IIPE P6 with 256KB of full-core speed L2 cache
Pentium IIIE P6 with SSE (MMX2) plus 256KB or 512KB of full-core speed L2 cache
Pentium III Xeon P6 with SSE (MMX2), 512KB, 1MB, or 2MB of full-core speed L2 cache

The main new feature in the fifth-generation Pentium processors was the superscalar architecture, in which two instruction execution units could execute instructions simultaneously in parallel. Later fifth-generation chips also added MMX technology to the mix. So then what did Intel add in the sixth generation to justify calling it a whole new generation of chip? Besides many minor improvements, the real key features of all sixth-generation processors are Dynamic Execution and the DIB architecture, plus a greatly improved superscalar design.

Pentium Pro Processors

Intel’s successor to the Pentium is called the Pentium Pro. The Pentium Pro was the first chip in the P6 or sixth-generation processor family. It was introduced in November 1995 and became widely available in 1996. The chip is a 387-pin unit that resides in Socket 8, so it is not pin compatible with earlier Pentiums. The chip is unique among processors because it is constructed in a multichip module (MCM) physical format, which Intel calls adual-cavity PGA package. Inside the 387-pin chip carrier are two dies. One contains the actual Pentium Pro processor, and the other contains a 256KB, 512KB, or 1MB L2 cache. (The Pentium Pro with 256KB cache is shown in Figure 3.22.) The processor die contains 5.5 million transistors, the 256KB cache die contains 15.5 million transistors, and the 512KB cache die(s) have 31 million transistors each—for a potential total of nearly 68 million transistors in a Pentium Pro with 1MB of internal cache! A Pentium Pro with 1MB cache has two 512KB cache die and a standard P6 processor die (see Figure 3.23). The incorporation of L2 cache is one of the most enduring legacies of the Pentium Pro because this feature has been incorporated into virtually every Intel and AMD processor built since, with the notable exception of the original Celeron.

Figure 3.22. Pentium Pro processor with 256KB L2 cache. (The cache is on the left side of the processor die.)
Photograph used by permission of Intel Corporation.

Pentium Pro Processor

Figure 3.23. Pentium Pro processor with 1MB L2 cache. (The cache is in the center and right portions of the die.) Photograph used by permission of Intel Corporation.

Pentium Pro Processor

Pentium II Processors

Intel revealed the Pentium II in May 1997. Prior to its official unveiling, the Pentium II processor was popularly referred to by its code name, Klamath, and was surrounded by much speculation throughout the industry. The Pentium II is essentially the same sixth-generation processor as the Pentium Pro, with MMX technology added (which included double the L1 cache and 57 new MMX instructions); however, there are a few twists to the design. The Pentium II processor die is shown in Figure 3.24.

Figure 3.24. Pentium II Processor die. Photograph used by permission of Intel Corporation.
Pentium II Processor die.

From a physical standpoint, it was a big departure from previous processors. Abandoning the chip in a socket approach used by virtually all processors up until this point, the Pentium II chip is characterized by its SEC cartridge (SECC) design. The processor, along with several L2 cache chips, is mounted on a small circuit board (much like an oversized-memory SIMM), as shown in Figure 3.25, and the circuit board is then sealed in a metal and plastic cartridge. The cartridge is then plugged into the motherboard through an edge connector called Slot 1, which looks much like an adapter card slot.

Figure 3.25. Pentium II processor board (normally found inside the SEC cartridge). Photograph used by permission of Intel Corporation.
Pentium II Processor Board Normally found inside the SEC cartridge

The two variations on these cartridges are called SECC (single edge contact cartridge) and SECC2.

The SECC2 version was cheaper to make because it uses fewer overall parts. It also allowed for a more direct heatsink attachment to the processor for better cooling. Intel transitioned from SECC to SECC2 in the beginning of 1999; all later PII chips, and the Slot 1 PIII chips that followed, use the improved SECC2 design.

By using separate chips mounted on a circuit board, Intel could build the Pentium II much less expensively than the multiple die within a package used in the Pentium Pro. Intel could also use cache chips from other manufacturers and more easily vary the amount of cache in future processors compared to the Pentium Pro design.

Intel offered Pentium II processors with the speeds listed in Table 3.14.

Table 3.14. Speeds for Pentium II Processors and Motherboards

CPU Type/Speed CPU Clock Motherboard Speed
Pentium II 233MHz 3.5x 66MHz
Pentium II 266MHz 4x 66MHz
Pentium II 300MHz 4.5x 66MHz
Pentium II 333MHz 5x 66MHz
Pentium II 350MHz 3.5x 100MHz
Pentium II 400MHz 4x 100MHz
Pentium II 450MHz 4.5x 100MHz

Aside from speed, the best way to think of the Pentium II is as a Pentium Pro with MMX technology instructions and a slightly modified cache design. It has the same multiprocessor scalability as the Pentium Pro, as well as the integrated L2 cache. The 57 new multimedia-related instructions carried over from the MMX processors and the capability to process repetitive loop commands more efficiently are included as well. Also included as a part of the MMX upgrade is double the internal L1 cache from the Pentium Pro (from 16KB total to 32KB total in the Pentium II).

For more information about the Pentium II, see Chapter 3 of Upgrading and Repairing PCs, 19th edition, available in its entirety on the disc packaged with this book.

Pentium III

The Pentium III processor, shown in Figure 3.26, was released in February 1999 and introduced several new features to the P6 family. It is essentially the same core as a Pentium II with the addition of SSE instructions and integrated on-die L2 cache in the later versions. SSE consists of 70 new instructions that dramatically enhance the performance and possibilities of advanced imaging, 3D, streaming audio, video, and speech-recognition applications.

Figure 3.26. Pentium III processor in SECC2 (Slot 1) and FC-PGA (Socket 370) packages.

Pentium III Processor in SECC2 Slot 1 and FC-PGA Socket 370 packages.

Originally based on Intel’s advanced 0.25-micron CMOS process technology, the PIII core started out with more than 9.5 million transistors. In late 1999, Intel shifted to a 0.18-micron process die (code-named Coppermine) and added 256KB of on-die L2 cache, which brought the transistor count to 28.1 million. The last version of the Pentium III (code-named Tualatin) uses a 0.13-micron process and has 44 million transistors; motherboards made before the Tualatin-core versions of the Pentium III generally do not support this processor because of logical pinout changes. The Pentium III was manufactured in speeds from 450MHz through 1.4GHz, as well as in server versions with larger or faster cache known as the Pentium Xeon. The Pentium III also incorporates advanced features such as a 32KB L1 cache and either half-core speed 512KB L2 cache or full-core speed on-die 256KB or 512KB L2 with cacheability for up to 4GB of addressable memory space. The PIII also can be used in dual-processing systems with up to 64GB of physical memory. A self-reportable processor serial number gives security, authentication, and system management applications a powerful new tool for identifying individual systems. Because of privacy concerns when the processor was released, you can disable this feature in the system BIOS on most systems that use the Pentium III or Celeron III processors.

Pentium III processors were made available in Intel’s SECC2 form factor, which replaced the more expensive older SECC packaging. The SECC2 package covers only one side of the chip and allows for better heatsink attachment and less overall weight. Architectural features of the Pentium III processor include the following:

  • Streaming SIMD extensions (SSE)— Seventy new instructions for dramatically faster processing and improved imaging, 3D streaming audio and video, web access, speech recognition, new user interfaces, and other graphics and sound-rich applications.

  • Intel processor serial number— Serves as an electronic serial number for the processor and, by extension, its system or user. You can enable or disable this feature as desired in the BIOS Setup. The serial number enables the system/user to be identified by company internal networks and applications. You can use the processor serial number in applications that benefit from stronger forms of system and user identification, such as the following:

    • Applications using security capabilities— Managed access to new Internet content and services; electronic document exchange.

    • Manageability applications— Asset management; remote system load and configuration.

Although the initial release of Pentium III processors was made in the improved SECC2 packaging, Intel later switched to the FC-PGA package, which is even less expensive to produce and enables a more direct attachment of the heatsink to the processor core for better cooling. The FC-PGA version plugs into Socket 370 but can be used in Slot 1 with a slotket adapter.

All Pentium III processors have either 512KB or 256KB of L2 cache, which runs at either half-core or full-core speed. Pentium III Xeon versions have 512KB, 1MB, or 2MB of L2 cache that runs at full-core speed. The Pentium III Xeon is a more expensive version of the Pentium III designed for servers and workstations. All PIII processor L2 caches can cache up to 4GB of addressable memory space and include ECC capability.


The Celeron processor is a chameleon, more of a marketing name than the name of an actual chip. In its first two versions it was originally a P6 with the same processor core as the Pentium II; later it came with the same core as the PIII, then the P4, while more recent versions use the same core as the Core 2 processors. The Celeron name represents essentially a version of Intel’s current mainstream chip that Intel has repackaged for lower-cost PCs.

In creating the original Celerons, Intel figured that by taking a Pentium II and deleting the separate L2 cache chips mounted inside the processor cartridge (and deleting the cosmetic cover), it could create a “new” processor that was basically just a slower version of the Pentium II. As such, the first 266MHz and 300MHz Celeron models didn’t include L2 cache. Unfortunately, this proved to have far too great a crippling effect on performance, so starting with the 300A versions, the Celeron received 128KB of on-die full-speed L2 cache, which was actually faster and more advanced than the 512KB of half-speed cache used in the Pentium II it was based on at the time! In fact, the Celeron was the first PC processor to receive on-die L2 cache. It wasn’t until the Coppermine version of the Pentium III appeared that on-die L2 cache migrated to Intel’s main processors.

Needless to say, this caused a lot of confusion in the marketplace about the Celeron. Considering that the Celeron started out as a “crippled” Pentium II and then was revised to actually be superior in some ways to the Pentium II on which it was based (all while selling for less), many didn’t know just where the Celeron stood in terms of performance. Fortunately, the crippling lack of L2 cache existed only in the earliest Celeron versions; all of those at speeds greater than 300MHz have on-die full-speed L2 cache.

Since then, the Celeron has been released in many different versions, with each newer one based on the then-current mainstream processor. The latest Celerons use the same basic 45nm “Wolfdale” core as more expensive Core 2 processors. The difference is that the Celeron versions are offered in lower processor and bus clock speeds and with smaller caches, to justify a lower price point.

Because Intel has offered Celeron and Celeron D processors in many distinctive variations, it’s easy to get confused as to which is which, or which is available at a specific speed. By identifying the spec number of a particular chip and looking up the number on the Intel product information website (, you can find out the exact specification, including socket type, voltage, stepping, cache size, and other information. If you don’t know the spec number, you can still look up the processor by the model number, or you can use software such as CPU-Z ( to find more detailed information about the processor.

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