![]() This gives you a very fast memory bus, along with the same 100MHz processor speed, as if you were running the chip in a 33/100MHz tripled mode. One interesting capability here is to run the DX4-100 chip in a doubled mode with a 50MHz motherboard speed. The motherboard documentation should cover these settings if they can be changed. In most cases, one or two jumpers will be on the board near the processor socket to control the settings for these pins. The internal multiplier of the DX4 processor is controlled by the CLKMUL (clock multiplier) signal at pin R-17 (Socket 1) or S-18 (Socket 2, 3, or 6). Intel DX2 and DX4 Operating Speeds Versus CPU Bus (Motherboard) Clock Speeds View Table Table 3.24 shows the various speed combinations that can result from using the DX2 or DX4 processors with different motherboard clock speeds. Note that the DX2/OverDrive processors operate internally at two times the motherboard clock rate, whereas the DX4 processors operate at two, two-and-one-half, or three times the motherboard clock rate. A 100MHz-rated 486DX4 chip, for example, runs at 75MHz if it is plugged into a 25MHz motherboard. The DX4 is different because it requires 3.3V to operate instead of 5V, like most other chips up to that time.Ī processor rated for a given speed always functions at any of the lower speeds. Most of the later 486-based motherboards with a single ZIF socket support any of the 486 processors except the DX4. If your motherboard has two sockets, the primary one likely supports the standard 168-pin configuration, and the secondary (OverDrive) socket supports the 169-pin OverDrive configuration. The DX, DX2, and SX processors have a virtually identical 168-pin configuration, whereas the OverDrive chips have either the standard 168-pin configuration or a specially modified 169-pin OverDrive (sometimes also called 487SX) configuration. Additionally, 486 processors have slight differences in overall pin configurations. Most of the 486 chips were offered in a variety of maximum speed ratings, varying from 16MHz up to 133MHz. You can see why the arrival of the 486 rapidly killed off the 386 in the marketplace. This made the 486 a much more desirable option, primarily because it could more easily be upgraded to a DX2 or DX4 processor at a later time. The 486 chip is about twice as fast as the 386, so a 386DX-40 is about as fast as a 486SX-20. On average, the math coprocessor built into the DX-series chips provides two to three times greater math performance than an external 387 chip. The math coprocessor runs synchronously with the main processor and executes math instructions in fewer cycles than previous designs did. ![]()
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