Intel Pentium

The Intel Pentium was one of the most iconic processors ever released for consumers and corporate customers, released in speeds from 60 MHz to 200 MHz.

History
Intel's new fifth-generation chip was expected to be called the 586, following their earlier naming conventions. However, with the rise of AMD and Cyrix, Intel wanted to be able to register as a trademark the name of their new CPU, as numbers cannot be trademarked. Thus, the name Pentium was born. It is now one of the most recognized trademarks in the computer world, although Intel has more or less now moved on from using it.

The Pentium was the defining processor of the fifth-generation. It has, in fact, had several generations itself. The first Pentium CPUs were different, in many ways, compared to the later ones. It had been the target for compatibility for AMD's K5 and Cyrix's 6x86 processors, as well as generations that have followed. The chip itself is instruction set compatible with earlier x86 CPUs, although it does include a few new (albeit rarely used) instructions.

The Pentium provided greatly increased performance over the Intel 80486 processors that preceded it, due to several architectural changes. Roughly speaking, a Pentium CPU is double the efficiency of an 80486 CPU at the same clock speed. In addition, the Pentium goes to much higher clock speeds than the 486 could ever achieve. The following are the key architectural enhancements made in the Pentium over the 80486-era CPUs (note that some of these are present in Cyrix's 5x86 processor, but that chip was developed after the Pentium).

Superscalar Architecture
The Pentium was the first superscalar processor; it used two parallel execution units. Some people likened the Pentium to being like a pair of 80486s in the same chip for this reason, though this isn't totally accurate. It is really only partially superscalar, because the second execution unit doesn't have all the capabilities of the first; some instructions won't run in the second pipeline. In order to take advantage of the dual pipelines, code must be optimized to arrange the instructions in a way that will let both pipelines run at the same time. This is why you sometimes see reference to "Pentium optimization". Regardless, the performance is much higher than the single pipeline of the 80486.

Wider Data Bus
The Pentium CPU data bus is doubled to 64-bit, providing double the bandwidth for transfers to and from memory, compared to the 80486.

Much Faster Memory Bus
Most Pentium CPUs run on 60 or 66 MHz system buses while most 80486s run on 33 MHz system buses. This greatly improves performance. Pentium motherboards also incorporate other performance-enhancing features, such as pipelined burst cache. The Pentium processor was also the first CPU specifically designed to work with the (then-new) PCI bus.

Branch Prediction
The Pentium uses branch prediction, to prevent pipeline stalls when branches are encountered.

Integrated Power Management
All Pentium CPUs have built in SMM power management (which was optional on most of the 80486 CPUs).

Split Level 1 Cache
The Pentium uses a split Level 1 cache, 8 KiB each for data and instructions. The cache was split so that the data and instruction caches could be individually tuned for specific uses.

Improved Floating Point Unit
The floating point unit of the Intel Pentium is significantly faster than that of the 80486.

Availability
The Pentium was available in a wide variety of speeds, as well as in regular and OverDrive versions. It was also available in several packaging styles, although the Pin Grid Array (PGA) was the most prevalent. The original Pentium, the 60 and 66 MHz versions, were very different than the later versions that ware used in PCs afterward; they used older, 5-volt technology and encountered significant problems with heat dissipation. Intel solved this with later (75 to 200 MHz) versions by going to a smaller circuit size and a 3.3-volt power supply.

The Pentium CPUs used three different sockets. The original 60 and 66 MHz versions used Socket 4, Pentium CPUs from 75 MHz to 133 MHz can use either Socket 5 or Socket 7, while Pentium CPUs in 150, 166, and 200 MHz flavours required Socket 7. Intel made Pentium OverDrive adaptors that allowed the use of faster Pentium CPUs in older Pentium sockets (in addition to OverDrives that go in 80486 motherboards).

The Pentium processor achieved a certain level of "fame" as a result of a bug that was discovered in its floating point unit not long after it was released. This is commonly known as the "FDIV bug" after the instruction (Floating Point Divide) that it most commonly turns up in. While bugs in processors are relatively common, they usually are minor and don't have a direct impact on computation results. This one did, and achieved great notoriety in part because Intel didn't own up to the problem and offer to correct it immediately. Intel did eventually offer a replacement on affected processors, which were only found in early versions (60 to 100 MHz) sold in 1994 and earlier years.

If you suspect your Pentium of having the FDIV bug, try this computation test using a spreadsheet or calculator program: take the number 4,195,835 and divide it by 3,145,727. Then, take the result and multiply it by the same number again (3,145,727). You should of course get the same 4,195,835 back that you started with. On a PC with the FDIV bug, you will get 4,195,579 (an error of 256), but beware that some operating systems and applications have been patched to compensate for this bug, so a simple math test isn't necessarily conclusive.

For many years, the Pentium processor was the mainstream processor of choice, but finally the Pentium MMX CPUs drove it to the economy market. With the regular Pentium maxing out at 200 MHz and the Pentium MMX 166 MHz dropping well below $200, the "Pentium Classic" didn't make nearly as much sense as it used to for new PCs. The 60 and 66 MHz versions soon became obsolete, due to their slow speed and older technology, and the 75 to 150 MHz versions went obsolete soon after, because their performance was much lower than the 166 MHz and 200 MHz versions that shipped for almost the same amount of money.

The entire Pentium line became obsolete quite quickly, firstly due to the availability of inexpensive, faster Pentium MMX chips (as well as comparable offerings at the time from AMD and Cyrix). The non-MMX Pentium was soon no longer used in new systems. However, since the Pentium MMX required split rail voltage, the classic Pentium 200 MHz remained a great chip for those who had Socket 7 motherboards and who wanted to upgrade, but who did not have split rail voltage support.