Pentium OverDrive - 486 Sockets

486 Sockets

The Pentium OverDrive was claimed to enable owners of 486 type motherboards to upgrade their machines to Pentium performance, without the cost of having to replace the entire system. The chip was a heavily modified Pentium P54 architecture, made with 0.6 micrometre technology and operating on 3.3 volts, but with a half-wide data bus (32-bit) and a larger 32 KB (16 KB + 16 KB) L1 cache, double its P5-platform Pentium peers. Unfortunately the design was plagued with various compatibility problems. Intel changed the specification during development, rendering previously-compatible motherboard designs incompatible. The chip also did not always benefit from the motherboard's cache RAM, resulting in sub-par performance.

When the Pentium OverDrive 83 MHz launched, significantly later than the mere 63 MHz version, it did so at $299, an exorbitant price compared to other upgrade alternatives. The AMD 5x86 and Cyrix Cx5x86 processors were usually faster and were considerably cheaper. Even Intel's own DX4, based on an older chip architecture, was typically faster. Only on some applications, where floating point arithmetic was used, could the Pentium OverDrive outperform its predecessors.

Two interesting parts of the Pentium OverDrive for 486 systems are the integrated fan/heatsink combination and the onboard voltage regulation. The processor cooler is permanently attached and the fan is powered by a trio of conductors on the surface of the chip. They power the fan through spring-loaded metal points in the fan assembly, which is removable to allow replacement of the fan if necessary. The clip that releases the fan is viewable in the photo above, at the top left corner of the CPU. The central plastic "column" that leads from the center of the fan houses the fan wiring and leads down the side of the heatsink at this corner. The small plastic points at each top left of this column are the locking mechanism for the fan and are released by squeezing them. The opposite corner of the CPU has a latch that locks the fan around underneath the heatsink, by swinging into place upon assembly. The processor monitors the fan and will throttle back on clock speed to prevent overheating and damage if the fan is not operating. This is a predecessor to the internal temperature detection and protection in Intel's modern processors.

The onboard power regulation circuitry, partly visible near the bottom of the photo, allows the CPU to operate on boards that provide only 5 volts to the CPU. This is necessary because the processor (die) itself operates at 3.3 V like a regular P54C-core Pentium. Late-model 486 motherboards did provide this voltage, because some late-model 486 CPUs like the AMD 5x86 required it, but many boards only provided 5 V power.

PODP5V63

• Introduced February 3, 1995
• 235 pins, P24T pinout
• 5 or 3.3 volts
• L1 Cache 32 KB (16 KB + 16 KB)
• 63 MHz on 25 MHz front side bus (25 × 2.5)

PODP5V83

• Introduced October 1995
• 237 pins, P24T pinout
• 5 or 3.3 volts
• L1 Cache 32 KB (16 KB + 16 KB)
• 83 MHz on 33 MHz front side bus (33 × 2.5)