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 below shows the variations in the P6 family of processors.

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, as well.

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 Dual Independent Bus (DIB) architecture, plus a greatly improved superscalar design.

Dynamic Execution

Dynamic execution enables the processor to execute more instructions on parallel, so tasks are completed more quickly. This technology innovation is comprised of three main elements:

• Multiple branch prediction. Predict the flow of the program through several branches

• Dataflow analysis. Schedules instructions to be executed when ready, independent of their order in the original program

• Speculative execution. Increases the rate of execution by looking ahead of the program counter and executing instructions that are likely to be necessary

Dual Independent Bus

The other main P6 architecture feature is known as the Dual Independent Bus. This refers to the fact that the processor has two data buses: one for the system (motherboard) and the other just for cache. This enables the cache memory to run at speeds previously not possible.

Other Sixth-Generation Improvements

Finally, the P6 architecture upgrades the superscalar architecture of the P5 processors by adding more instruction execution units and by breaking down the instructions into special micro-ops. This is where the CISC instructions are broken down into more RISC commands.

The RISC-level commands are smaller and easier for the parallel instruction units to execute more efficiently. With this design, Intel has brought the benefits of a RISC processor—high-speed dedicated instruction execution—to the CISC world.

Note that the P5 had only two instruction units, whereas the P6 has at least six separate dedicated instruction units. It is said to be three-way superscalar because the multiple instruction units can execute up to three instructions in one cycle.

Other improvements in efficiency also are included in the P6 architecture: built-in multiprocessor support, enhanced error detection and correction circuitry, and optimization for 32-bit software.

Rather than just being a faster Pentium, the Pentium Pro, Pentium II/III, and other sixth-generation processors have many feature and architectural improvements. The core of the chip is very RISC-like, whereas the external instruction interface is classic Intel CISC.

By breaking down the CISC instructions into several RISC instructions and running them down parallel execution pipelines, the overall performance is increased. Compared to a Pentium at the same clock speed, the P6 processors are faster—as long as you're running 32-bit software.

The P6 Dynamic Execution is optimized for performance primarily when running 32-bit software, such as Windows NT. If you are using 16-bit software, such as Windows 95 or 98 (which still operate part time in a 16-bit environment) and most older applications, the P6 does not provide as marked a performance improvement over similarly speed-rated Pentium and Pentium MMX processors.

That's because the Dynamic Execution capability is not fully exploited. Because of this, Windows NT/2000/XP often are regarded as the most desirable operating systems for use with Pentium Pro/II/III/Celeron processors. Although this is not exactly true (a Pentium Pro/II/III/Celeron runs fine under Windows 95/98), Windows NT/2000/XP does take better advantage of the P6's capabilities.

Note that it is really not so much the operating system but which applications you use. Software developers can take steps to gain the full advantages of the sixth-generation processors.

This includes using modern compilers that can improve performance for all current Intel processors, writing 32-bit code where possible, and making code as predictable as possible to take advantage of the processor's Dynamic Execution multiple branch prediction capabilities.