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The Intel 80486SX is the same chip as the 80486DX with one exception: the lack of an integrated math coprocessor (floating point unit). Note that this is a different kind of difference between the SX and DX versions than is the case with the 386; the 386SX and 386DX both had no coprocessor and the 386SX had narrower data and address buses than the 386DX. The 486DX and 486SX have the same bus widths. Since it is the same chip except for the floating point processor, the 486SX has the same advantages over the 386 that the 486DX does. Note that the 486SX was made available in slower clock speeds than the 486DX; the SX comes in 16, 20, 25 and 33 MHz versions, while the DX is 25, 33 and 50 MHz.
Intel made some rather interesting (some would say bizarre) decisions about how to market the 486 line. The 486SX was in many ways a marketing gimmick only; it was in fact a 486DX with the math coprocessor disabled! Why did Intel bother to sell a full 486DX and a "crippled" one (the 486SX)? Mainly for market targeting reasons: they wanted to be able to fill the niche of low-end buyers without dropping the price on their "top of the line" 486DX. The 486SX was a popular choice for laptops due to its lower cost and lower power consumption.
The 80486SX uses a "math coprocessor" called the 80487SX (there is no 80487DX at all so don't be confused by that). The 80487SX is, in fact, a fully functional 80486DX chip! What Intel wanted people to think was that (like with its earlier coprocessors) you would put the 80487SX in and it would handle the math functions. In fact, when inserted, the 80487SX shuts down the 80486SX and handles both integer and floating point operations (since it is internally a 80486DX, which does both). This makes no difference from a performance standpoint but is kind of a technical curiosity.
With the large number of very cheap 486DX and faster chips and systems around, the 486SX is really quite obsolete. In addition, it does not come in a 50 MHz version. If you are not doing any floating point operations at all it is comparable to a 486DX of the same speed, but there is usually no need to have to worry about this given that even the 486DX is quite obsolete at this time anyway.
Note: The 486SX processor
normally was purchased as part of a new system only, not as part of an upgrade. Most early
486 systems used a 168-pin socket for the chip, which predates the numbered standardized
socket system that Intel created. The 486SX will fit into a Socket 1, Socket 2 or Socket 3
Look here for an explanation of the categories in the processor summary table below, including links to more detailed explanations.