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Intel 80486DX2 and 80486DX2 OverDrive

The 80486DX2 was the first chip to use "clock doubling" technology, where the processor runs at a faster speed than the memory bus it talks to. This was done to allow the processor speed to be increased without having to deal with the much more difficult task of increasing motherboard speed. Chips that run at faster than memory bus speed improve performance but at a diminishing rate as the multiplier increases, due to the processor waiting for data from memory. This is discussed in detail here.

Intel produced 50 and 66 MHz DX2 chips, intended for use in 25 and 33 MHz system bus systems. These chips have been sold as regular chips intended for use in new systems, which generally come in 168 pin packages to go in the original 168 pin socket used in 486 systems. They have also been made in 169 pin OverDrive versions to go in Socket 1 (the original OverDrive socket). These can be used to upgrade older 486DX or 486SX systems.

AMD and Cyrix not only cloned the 66 MHz DX2 processor, they took Intel one step further with the 80486DX2-80, running at 80 MHz. This uses a 40 MHz system bus, which isn't a speed that is normally used by Intel systems but that became more popular late in the 486 life cycle due to the performance increase it gives over 33 MHz bus systems. In addition, the AMD (enhanced version) and Cyrix chips have several advantages over the Intel chips (they had the benefit of developing them well after Intel):

• Power Management: They have SMM (power management) built in.
• Write-Back Cache: They have write-back capability for the primary cache which provides a small boost in performance.
• Lower Power: They use less power because they run at 3 volts, but will still work in standard 5 volt motherboard sockets without a converter or regulator (they are said to be "5 volt tolerant").

Other than clock speed, the 80486DX2 is virtually identical to the 80486DX. They are obsolete due to the availability of faster, very inexpensive processors such as the 5x86-133 that go in the same sockets, but 486DX2 systems are perfectly viable for many uses, including routine office word processing and spreadsheet work under DOS and Windows 3.x. The 80486DX2-66 is by far the most common version of this chip; a great number of these systems were produced and many are still in use today, especially in small businesses.

Note: The 486DX2 was the first processor to really require a heat sink in order to operate reliably. The increased speed of this chip means that it runs very hot (at least the Intel 5 volt versions).

Look here for an explanation of the categories in the processor summary table below, including links to more detailed explanations.

Next: Intel 80486DX4 and 80486DX4 OverDrive

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