What is the difference between 386sx, 386dx, 486sx, and 486dx processors?

All 386 and 486 processor models are "32-bit" CPUs. What, this means is that the processors' registers are large enough to load and store data, that is up to 32-bits in length. It does not matter if the processor model is designated SX or DX, it is still 32 bit.

One misconception about these processors is that their maximum memory addressability is only 4 Gigabytes. They can all address considerably more than that, up to 64 Terabytes. Most of it is virtual memory – memory that is not real, but simulated from disk or some other storage media.

Now for the differences. From software developer's point of view a 486 does not look very different from a 386, and it isn't. A 486 is an updated version of the 386, with considerably reengineered internal circuitry, which generally allows it to execute equivalent instructions faster. So given a 386 and 486 rated at the same clock frequency, the 486 will generally be faster. The extra speed of the 486 is due to its extra wide internal data pathways (128 bit vs. 32 bit for the 386), which allows blocks of data to flow back and

forth concurrently, instead of consecutively. There is also an internal ram cache, which is a buffer between the 486 and the slow external ram, which could cause the 486 to wait too long for memory access.

The difference between SX and DX depends entirely on whether you are talking about 386 or 486. In general, an SX is an economy version of the full DX processor, usually with some feature removed from it.

In the case of the 386, the SX and SL (an updated SX for use in low power notebook computers) versions have a smaller external data path; 16-bit vs. 32 bit for the DX. This smaller data path does not mean that the 386SX's 32-bit registers are disabled, it simply means that the SX has to take two passes to fill the same register. This is done automatically by the processor's own circuitry, and the software developer does not have to do anything different when writing software for 386SX that one wouldn't do with a 386DX. Naturally this can mean that a 386SX will run somewhat slower than a 386DX, given the same clock frequency. The 386SX also has had some of its address lines removed. This results in a lower physical memory addressibility, but its virtual memory addressability remains at 64 Terabytes. The Intel and clone 386SXs can address up to 16 Megabytes, the Intel 386SL can address 32 Megabytes, and the Intel and clone 386DX can address 4 Gigabytes physically.

IBM has a specialized version of the Intel 386SL called the 386SLC, which has added an internal 8K cache. It is otherwise pin compatible with the regular Intel 386SL.

AMD and C&T Corp both have clones of the Intel 386SX and 386DX. They are in general very much the same as their Intel counterparts and are direct replacements of each other, both in terms of pin compatibility, and software compatibility.

In the case of the 486, there are different variations within the SX theme. Intel, Cyrix, and IBM all have made different modifications to the 486DX design to come up with their economy 486.

The 486DX is distinguishable from all 386 and economy 486 offerings because it has an internal math coprocessor. The math coprocessor used to be a separate add-on extension to a processor, because ail its extra functions required more silicon than was available on the CPU. With increased miniaturization techniques, the 486DX was able to incorporate the circuitry directly into its own die.

Intel's variation of the economy 486 is called the 486SX. The 486SX has had its internal math coprocessor cut, but all data paths and address lines remain intact. The Intel 486SX and 486DX are not pin compatible with

each other, so one cannot simply remove a 486SX and place a 486DX in its place or vice-versa.

Cyrix has two levels of economy 486s: the 486SLC and the 486DLC. For the 486SLC, Cyrix has cut the math сорго too, but it has also reduced the size of the internal cache to IK from 8K, and repackaged the CPU to be pin-compatible with a 386SX CPU. Since it is pin compatible with an Intel 386SX, the 486SLC can only address 16Megs of memory, and it has a 16-bit external data path. Cyrix is positioning its offering as a direct replacement of the 386SX. For the 486DLC, all the same modifications that Cyrix made for the 486SLC apply, except that it is made¹ pin-compatible with an Intel 386DX rather than 386SX. Since the 486DLC is pin compatible with the Intel 386DX, it will be able to address 1 Gigabytes, and it has a 32-bit external data path. The Intel 486SX and the Cyrix 186XLC are not pin-compatible with each other, but they are software-compatible.

IBM has come out with its own variation of the 486, confusingly called the 486SLC. IBM's 486SLC is not pin-compatible with the Cyrix 486SLC, it is pin-compatible with the Intel 386SL/IBM 386SLC, however. The IBM 486SLC does not have a math сорго built-in, but it has a larger internal ram cache than all other 486s, 16K. Because of its 386SL/386SLC heritage, the 486SLC incorporates the same power-saving features, and it can address up to 32Megs, 16-bits at a time.

IX. Чтобы лучше понять различия между процессорами, заполните таблицу.