PENT CLASS CPUS

8/22/98

Pentium Class CPUs: The Pentium CPU was released by Intel in 1993 as a 5 volt CPU operating at 2 times the system bus speed nominally at 60 and 66MHz. It was packaged in a 273 pin PGA (Socket 4) package. These were followed in 1994-1997 with a series of 3.3volt (more or less) processors packaged in a 296 pin PGA (Socket 7 or the 320 pin Socket 5). The 3 volt CPUs (P54) operated at clock multiples of 1, 1.5. 2, 2.5 or 3 times and rated at 75, 90, 100, 120,133,150,166 and 200 MHz. In older motherboard machines the CPU clock was run externally at twice the 25, 30 or 33 MHz bus speed. In later designs, the bus speed was upped to 66MHz.

The name Pentium was chosen to avoid trademark protection problems that had allowed other vendors to sell 486s labeled as 486s. It has been retained for later, different, CPUs including the Pentium-Pro, Pentium MMX, Pentium II, and Pentium III -- which I will address in later articles.

The Pentium used the same memory architecture as the 486 and used 32 bit data internally. The bus width was expanded to 64 bits. The cache was 16KB implemented as 8K data and 8K instruction. All Pentiums have an internal floating point unit, and early versions were subject to the infamous "Pentium Bug" a problem that resulted in certain specific operands yielding slightly inaccurate division results.

The Pentium incorporated a second processing unit, but it was only usable on a small percentage of instructions. Intel advocated that programs be recompiled to optimize performance for the Pentium, but that rarely happened. It was estimated that performance could be improved by 50% by recompilation. Thanks to larger cache, some instruction timing improvements, the second processor, and wider bus width, normal (unrecompiled) code ran about 70 percent faster on a Pentium than a 486. Intel rates a 133MHz Pentium at 219MIPS implying something like 1.9 Instructions per clock cycle (Not very likely).

5 volt CPUs and high speed 3.3 volt CPUs require substantial heat removal. The P-66 (5-Volts) dissipated 16 watts. the P-75 which was designed for notebook applications dissipated only 3. Low speed 3.3 volt CPUs can be run with no or minimal heat sinking. Some Pentium motherboards suffered from inadequate heat sinking of the Voltage Regulators.

NexGen (later acquired by AMD), Cyrix(M1) and AMD(K6) all produced Pentium clones. In general, these had poorer floating point performance than Intel, but had significantly better integer performance than Intel Pentiums at the same CPU speed. This led to so called "P-ratings" where the CPU "speed" is the equivalent Intel CPU speed, not the actual speed the CPU runs at. Intel released Pentium overdrive CPUs with a 32K internal cache for the 486, but they were late to market, high priced, and did not perform well.

For reasons that possibly make sense to Intel, many of their Pentium CPUs do not have the speed indicated on them in any intelligible fashion.