Intel Pentium: A Bothersome Bug
The Pentium, introduced in 1993, was interesting for more than one reason. It was the first x86 to drop the traditional model number for a more attractive name, since Intel wasn’t allowed to trademark a name made up of numbers only. It’s also famous because of a bug it contained. On the first generations of Pentiums, certain division operations produced an incorrect result. Intel replaced the processors, but the damage was done. A very rare error gave rise to the first big IT media buzz.
The Pentium was sold in three different versions, the first without a CPU multiplier, the second with a multiplier (including the very familiar Pentium 166), and the last with the SIMD instruction set for x86s, MMX. The Pentium MMX also increased the size of the Level 1 cache and brought in a few minor improvements. This was the first Intel x86 capable of executing two instructions in parallel. The L2 cache was on the motherboard with these processors (running at the frequency of the FSB).
| Code name | P5, P54 | P55 (Pentium MMX) |
| Date released | 1993 | 1997 |
| Architecture | 32 bits | 32 bits |
| Data bus | 64 bits | 64 bits |
| Address bus | 32 bits | 32 bits |
| Maximum memory | 4096 MB | 4096 MB |
| L1 cache | 8 KB + 8 KB | 16 KB + 16 KB |
| L2 cache | Motherboard (FSB frequency) | Motherboard (FSB frequency) |
| Clock frequency | 60-200 MHz | 133-300 MHz |
| FSB | 50-66 MHz | 60-66 MHz |
| FPU | on chip | on chip |
| SIMD | no | MMX |
| Fabrication process | 800-600-350 nm | 350 nm |
| Number of transistors | 3.1-3.3 million | 4.5 million |
| Power consumption | 8-16 W | 4-17 W |
| Voltage | 5 V-3.3 V | 2.8 V |
| Die surface area | 294-163-90 mm² | 141 mm² |
| Connector | Socket 4, 5 or 7 | Socket 7 |
Here’s a little explanation of the Pentium bug: certain calculations using the FPU resulted in erroneous results. This was fairly rare—though sources disagree about exactly how rare—and Intel replaced the defective processors free of charge. Here’s an example of a Pentium error:
4195835.0/3145727.0 = 1.333 820 449 136 241 002 (correct result) 4195835.0/3145727.0 = 1.333 739 068 902 037 589 (incorrect result on a defective Pentium)
