Pentium Overdrive

Pentium Overdrive

The P24T Pentium OverDrive for 486-systems were released in 1995, which were based on 3.3V 0.6 µm versions using a 63 or 83 MHz clock. Since these used Socket 2/3, some adjustment had to be made to compensate for the 32-bit data bus and slower on-board L2 cache of 486-motherboards. They were therefore equipped with a 32KB L1 cache (double that of pre-P55C Pentiums). The product included an attached fan and heatsink assembly as well as power regulation to convert 5V down to 3.3V. Intel had promised 486 users an upgrade in the "near future" via an "Overdrive" socket to accommodate a Pentium-like CPU. However, the P24T arrived late, at low speeds, and high costs, which led some users to suspect that Intel never really planned such an upgrade, but eventually offered it to avoid a lawsuit. The P24Ts therefore sold poorly. In addition, motherboard compatibility with the P24T was poor, possibly due to the use of a write-back caching scheme. Common workarounds were to disable the motherboard's L2 cache causing a noticeable drop in system performance.

P55C, Tillamook
Pentium logo, with MMX enhancement
Intel Pentium MMX microarchitecture.
Pentium MMX 166 MHz without cover

The P55C (or 80503) was developed by Intel's Research & Development Center in Haifa, Israel. It was sold as Pentium with MMX Technology (usually just called Pentium MMX); although it was based on the P5 core (the 0.35 µm process was also used for this series) it featured a new set of 57 "MMX" instructions intended to improve performance on multimedia tasks, such as encoding and decoding digital media data.

The new instructions work on new data types: 64-bit packed vectors of either eight 8-bit integers, four 16-bit integers, two 32-bit integers, or one 64-bit integer. So, for example, the PADDUSB (Packed ADD Unsigned Saturated Byte) instruction adds two vectors, each containing eight 8-bit unsigned integers together, pairwise; each addition that would overflow saturates, yielding 255, the maximum unsigned value that can be represented in a byte. These rather specialized instructions generally require special coding by the programmer for them to be used. MMX did not achieve significant popularity until after the P55C's lifetime[citation needed]. The performance of the P55C was improved over previous versions by a doubling of the Level 1 CPU cache from 16 KiB to 32 KiB.

Pentium P55C notebook CPUs used a "mobile module" that held the CPU. This module was a PCB with the CPU directly attached to it in a special smaller form factor. The module snapped to the notebook motherboard and typically a heat spreader plate was installed and made contact with the module. Such notebooks frequently used the Intel 430MX chipset, a feature-reduced 430FX. However, with the 0.25 µm Tillamook Mobile Pentium MMX (named after a city in Oregon), the module also held the 430TX chipset along with the system's 512 KiB SRAM cache memory.