Gecko's CPU Library

Intel Pentium Pro (P6) processors

Introduction: November 1995

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 new chip is unique among processors as it is constructed in a Multi-Chip Module (MCM) physical format, which Intel is calling a Dual Cavity PGA (Pin Grid Array) package. Inside the 387-pin chip carrier are two dies. One contains the actual Pentium Pro processor, and the other a 256KB, 512KB, or 1MB L2 cache. 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. The main processor die includes a 16KB split L1 cache with an 8KB two-way set associative cache for primary instructions and an 8KB four-way set associative cache for data.

Another sixth-generation processor feature found in the Pentium Pro is the Dual Independent Bus (DIB) architecture, which addresses the memory bandwidth limitations of previous-generation processor architectures. Two buses make up the DIB architecture: the L2 cache bus (contained entirely within the processor package) and the processor-to-main memory system bus. The speed of the dedicated L2 cache bus on the Pentium Pro is equal to the full-core speed of the processor. This was accomplished by embedding the cache chips directly into the Pentium Pro package. The DIB processor bus architecture addresses processor-to-memory bus bandwidth limitations. It offers up to three times the performance bandwidth of the single-bus, "Socket 7" generation processors, such as the Pentium.

The integrated L2 cache is one of the really outstanding features of the Pentium Pro. By building the L2 cache into the CPU and getting it off the motherboard, the Pentium Pro can now run the cache at full processor speed rather than the slower 60 or 66MHz motherboard bus speeds. In fact, the L2 cache features its own internal 64-bit backside bus, which does not share time with the external 64-bit frontside bus used by the CPU. The internal registers and data paths are still 32-bit, as with the Pentium. By building the L2 cache into the system, motherboards can be cheaper because they no longer require separate cache memory. Some boards may still try to include cache memory in their design, but the general consensus is that L3 cache (as it would be called) would offer less improvement with the Pentium Pro than with the Pentium.

One of the features of the built-in L2 cache is that multiprocessing is greatly improved. Rather than just SMP, as with the Pentium, the Pentium Pro supports a new type of multiprocessor configuration called the Multiprocessor Specification (MPS 1.1). The Pentium Pro with MPS allows configurations of up to four processors running together. Unlike other multiprocessor configurations, the Pentium Pro avoids cache coherency problems because each chip maintains a separate L1 and L2 cache internally.

Source: Upgrading and Repairing PCs (13th Edition) by Scott Mueller.